Aurora M. Nedelcu, Department of Biology, University of New Brunswick, PO Box 4400, Fredericton, NB E3B 5A3, Canada. Tel.: +506 458 7463; fax: +506 453 3583; e-mail: email@example.com
Although lateral gene transfer (LGT) is now recognized as a major force in the evolution of prokaryotes, the contribution of LGT to the evolution and diversification of eukaryotes is less understood. Notably, transfers of complete pathways are believed to be less likely between eukaryotes, because the successful transfer of a pathway requires the physical clustering of functionally related genes. Here, we report that in one of the closest unicellular relatives of animals, the choanoflagellate, Monosiga, three genes whose products work together in the glutamate synthase cycle are of algal origin. The concerted retention of these three independently acquired genes is best explained as the consequence of a series of adaptive replacement events. More generally, this study argues that (i) eukaryote-to-eukaryote transfers of entire metabolic pathways are possible, (ii) adaptive functional replacements of primary pathways can occur, and (iii) functional replacements involving eukaryotic genes are likely to have also contributed to the evolution of eukaryotes. Lastly, these data underscore the potential contribution of algal genes to the evolution of nonphotosynthetic lineages.
Lateral gene transfer (LGT) is currently recognized as a major force in the evolution of prokaryotes (e.g. Boucher et al., 2003). However, with the exception of the massive transfers associated with the establishment of mitochondria and plastids, the contribution of LGT (especially eukaryote-to-eukaryote transfer) to the evolution and diversification of eukaryotic lineages is less understood (Andersson, 2005; Keeling & Palmer, 2008). Laterally acquired genes can be added to the recipient’s gene complement (gene additions) or simply replace existing endogenous counterparts (functional replacements). The recruitment of novel genes is thought to allow the recipient to adapt to specialized or new ecological niches (such as anaerobic or sugar-rich environments, soil, etc.) as well as new life-styles (parasitism) (e.g. Andersson et al., 2003; Opperdoes & Michels, 2007; Keeling & Palmer, 2008). On the other hand, as functional replacements can be either adaptive or selectively neutral, their impact on the recipient’s adaptive or long-term evolutionary potential is less clear (e.g. Huang & Gogarten, 2008; Keeling & Palmer, 2008).
Although most LGTs involve single genes that can function independently or, if replacing a homologue, become part of a mosaic/chimeric pathway [e.g. the carotenoid biosynthetic pathway in chromist algae; (Frommolt et al., 2008)], complete pathways have also been transferred between prokaryotes as well as from prokaryotes to eukaryotes [e.g. the transfer of a pathway involved in vitamin B6 biosynthesis, from a prokaryotic source to a nematode; (Craig et al., 2008)]. However, most of the recruited bacterial operons involve novel genes associated with nonessential pathways that have the potential to provide the recipient with new adaptive capabilities; the replacement of complete primary metabolic pathways is known to occur only under special environmental conditions [e.g. the acquisition of the arginine biosynthesis operon in Xantomonadales; (Lima & Menck, 2008)]. Notably, transfers of complete pathways (either additions or functional replacements) are believed to be less likely between eukaryotes, because the successful transfer of a complete pathway requires the physical clustering of functionally related genes (such as in operons), and – in contrast to prokaryotes, such clustering is rather infrequent among eukaryotes (Lawrence & Roth, 1996).
To address the impact of eukaryote-to-eukaryote LGT on the long-term evolution and diversification of eukaryotic lineages, we are investigating the potential contribution of algal genes to the evolution of choanoflagellates, a strictly nonphotosynthetic group that comprises both unicellular and colonial forms (Carr et al., 2008). Sequence data for two unicellular choanoflagellate species, Monosiga brevicollis and Monosiga ovata, are available (http://genome.jgi-psf.org/Monbr1/; http://amoebidia.bcm.umontreal.ca/pepdb/). The two Monosiga species inhabit distinct habitats (marine vs. freshwater), and each belongs to one of the three major clades currently recognized within Choanoflagellata (Carr et al., 2008). Phylogenetic analyses using various types of sequences (single genes or concatenated) as well as a number of molecular traits (i.e. metazoan-specific proteins) support the notion that Choanoflagellata is the sister taxon to animals (Metazoa), and that choanoflagellates, animals and fungi form a well-supported monophyletic group, the Opisthokonta, to the exclusion of all photosynthetic lineages (e.g. Cavalier-Smith & Chao, 2003; Nozaki et al., 2007; Abedin & King, 2008; Carr et al., 2008; King et al., 2008; Ruiz-Trillo et al., 2008).
Recently, we reported the presence of several stress-related genes of algal origin (including two ascorbate peroxidase and two metacaspase genes) in Monosiga (Nedelcu et al., 2008). Because at least three of these genes represent additions to the choanoflagellate gene complement and appear to have been acquired early in the evolution of choanoflagellates (i.e. before the divergence of the lineages leading to the extant M. brevicollis and M. ovata), we suggested that they could have contributed to this group’s ability to adapt to new environments (e.g. freshwater) and/or new life-styles (e.g. sessile, colonial). Here, we are addressing the potential contribution of a different class of genes, namely, genes involved in a primary metabolic pathway, and a different evolutionary outcome – i.e. functional gene replacement.
In addition to their ability to capture light, photosynthetic organisms differ from animals in their capacity to acquire and utilize nitrogen. Nitrogen is an indispensable element that is incorporated in many important structural and functional molecules, including amino acids and nucleotides. While algae and plants can synthesize organic nitrogen from inorganic sources (such as nitrite, nitrate and ammonium), animals are dependent on organic nitrogen produced by autotrophs (Inokuchi et al., 2002; Katagiri & Nakamura, 2003). Ammonium plays a central role in the metabolism of nitrogen, in both autotrophs and heterotrophs. The main route of ammonium assimilation (either directly up-taken from the environment, synthesized from nitrates, or metabolically-produced) is the so-called glutamine synthetase/glutamate synthase cycle (Lea et al., 1990) Fig. 1). In the first reaction, catalyzed by glutamine synthetase (GS), glutamine is synthesized via the ATP-dependent condensation of ammonium with glutamate. The second reaction, catalyzed by glutamate synthase (GLTS or GOGAT), ensures the regeneration of glutamate via the transfer of the amido group from glutamine to 2-oxoglutarate, to yield two molecules of glutamate. The products of this central cycle (aka the GS/GOGAT or the glutamate synthase cycle) serve as the starting points for the synthesis of all other nitrogen compounds, via a series of aminotransferase reactions (Fig. 1).
The GS/GOGAT cycle is found across all three domains of life, and thus is believed to be a very old pathway (e.g. Raymond, 2005). However, despite its central role in nitrogen metabolism, including amino acid biosynthesis and the neutralization of excess catabolic ammonium, vertebrates appear to have lost the glutamate synthase component. Indeed, while GLTS sequences are present in invertebrates and early chordates such as cephalochordates, no GLTS homologues have been found among vertebrates. The loss of GLTS in vertebrates is likely related to the fact that animals, in contrast to plants, can regenerate the glutamate needed for the first reaction of the GS/GOGAT cycle from dietary nitrogen-containing precursors (Katagiri & Nakamura, 2003).
Interestingly, although nitrogen is a major nutrient for all organisms, its availability is inadequate in many environments (Raymond, 2005). Furthermore, although an important plant nutrient and the preferred nitrogen source (partly because of the lower energetic cost to metabolize ammonium relative to nitrate), ammonium is often limiting for optimal plant growth; in fact, in most soils and in most coastal waters nitrate is more abundant than ammonium (see Inokuchi et al., 2002; Gonzalez-Ballester et al., 2004 for discussion and references). The large nitrogen requirement of plants resulted in their evolving unique strategies to acquire, capture and/or release ammonium, including a number of high-affinity ammonium transporters that belong to the AMT/Rh family (von Wiren et al., 2000; Ludewig et al., 2007). Notably, while present in invertebrates, AMT transporters are missing in vertebrates, which posses a distantly related family of transporters – the so-called Rhesus (Rh) glycoproteins (Huang & Peng, 2005). As in the GLTS case discussed above, the loss of AMTs in vertebrates might be because of their specific metabolic regimes; this is consistent with the loss of AMTs in species that live in nitrogen-rich environments, such as the unicellular parasites, Plasmodium falciparum and Trypanosoma brucei (Ludewig et al., 2007). Alternatively, the loss may have occurred because the extremely toxic ammonium derived from amino acid catabolism is salvaged and reused by reversing the glutamate dehydrogenase reaction (Huang & Peng, 2005).
We have searched the available genome and EST Monosiga databases for sequences encoding components of the GS/GOGAT cycle and ammonium transporters. Here, we report the finding of GS, GLTS and AMT genes of algal origin in Monosiga. The independent acquisition of these sequences whose products work together in a central metabolic pathway, the GS/GOGAT cycle, is best explained as the consequence of a series of adaptive replacement events. To our knowledge, this is the first example of functional replacement of a complete primary metabolic pathway involving eukaryote-to-eukaryote gene transfer. Overall, these findings argue that (i) entire pathways can be acquired in the absence of physical clustering of the corresponding genes, (ii) adaptive functional replacement of primary metabolic pathways can occur via multiple independent gene acquisitions and selective retention events, and (iii) functional replacements involving eukaryotic genes are also likely to have contributed to the evolution and diversification of eukaryotes.
Three main classes of evolutionarily related GLTSs are currently recognized (e.g. Temple et al., 1998; Raymond, 2005; Suzuki & Knaff, 2005; Vanoni et al., 2005). Most eubacteria possess a NADPH-dependent GLTS (EC 184.108.40.206) consisting of two distinct subunits, alpha and beta. On the other hand, eukaryotes (including plants, fungi and invertebrates) express a NADH-dependent GLTS (EC 220.127.116.11) comprising a single long polypeptide derived from the fusion of the bacterial alpha and beta subunits (Andersson & Roger, 2002). However, cyanobacteria and plants (in their chloroplasts) use an additional ferredoxin-dependent GLTS (EC 18.104.22.168), represented by a single polypeptide chain similar in size and sequence to the alpha subunit of the eubacterial NADPH.
Our searches in the Monosiga genome and EST databases identified a GLTS gene in the M. brevicollis genome. The structure of this GLTS gene, covering both the alpha and beta subunits, indicates that the encoded protein is a NADH-dependent GLTS. Phylogenetic analyses (both Bayesian and maximum likelihood) including NADH-, NADPH-, and Ferredoxin-dependent GLTSs from all major lineages confirmed the inclusion of the M. brevicollis predicted protein among NADH-dependent GLTSs, but failed to cluster this GLTS with its animal and fungal homologues. Instead, the M. brevicollis GLTS branched consistently, and with strong support, with the green algal/land plant group (Fig. 2a). Consistent with a green algal affiliation, the location of several insertions is shared between M. brevicollis and green algal/plant GLTS sequences, to the exclusion of animal and fungal homologues (Fig. 2b) (note that there are no insertions that are uniquely shared by M. brevicollis and animals and/or fungi; see Fig. S1 for a full alignment). Altogether, these findings argue strongly that the M. brevicollis GLTS gene has been acquired laterally, from an algal donor. In the absence of genomic information for M. ovata, the timing of this event – i.e. before or after the divergence of the two Monosiga lineages, cannot be inferred at this time.
Glutamine synthetases (E.C. 22.214.171.124) are coded by three distinct gene families, GSI/GSII/GSIII, and the number and type of GS isoenzymes vary greatly among lineages (e.g. Robertson et al., 2001; Raymond, 2005; Robertson & Tartar, 2006). For instance, among eukaryotes, Opisthokonta (fungi and animals) and Plantae (glaucophytes, red and green algae, and land plants) possess GS enzymes of the type II, while lineages within Chromalveolata (i.e. diatoms, oomycetes, haptophytes) have GSIII and/or GSII enzymes.
Our searches in the available Monosiga databases identified a GS gene in the M. brevicollis genome and a partial GS sequence in the M. ovata EST database; both predicted GS enzymes are of the type II and cluster together (see Fig. S2). However, phylogenetic analyses failed to group the predicted Monosiga GS proteins with homologues from their closest relatives, the animals (Fig. 3a and Fig. S2). Instead, the two Monosiga GSII sequences consistently grouped within the clade of photosynthetic (and previously photosynthetic) GSII homologues; specifically, at the base of (or within) the chromalveolate clade (in Bayesian analyses; Fig. 3a) or – with less support, at the base of the Plantae/Chromalveolata clade, close to the glaucophyte GSII sequence (in some maximum likelihood analyses; data not shown). Overall, although with variable support, in all analyses, regardless of method, phylogenetic distribution of the taxa included (e.g. with or without prokaryotic sequences) and number of taxa or sites, Monosiga GSII sequences grouped with counterparts from photosynthetic lineages, and away from fungal and animal homologues.
In this context, it should be mentioned that most photosynthetic eukaryotes have both cytosol and plastid-targeted GS isoenzymes. However, while in vascular plants, both cytosolic and chloroplast enzymes are of the GSII type [and arose via a recent duplication event in the plant lineage; (Coruzzi et al., 1989)], in diatoms, the cytosolic and plastid-targeted GSs are members of the GSIII and GSII families respectively, with the latter believed to be the result of an endosymbiotic gene transfer from the nuclear genome of the red algal symbiont that gave rise to the diatom plastid (Robertson et al., 2001; Robertson & Tartar, 2006). Interestingly, although GSIII sequences are also known from other chromalveolates (i.e. haptophytes) as well as amoebozoans, the secondarily nonphotosynthetic relatives of diatoms, the oomycetes, possess only GSII sequences; the absence of the ancestral GSIII gene in oomycetes is believed to be the consequence of a functional replacement by the endosymbiont-derived (nuclear-encoded) GSII gene (Robertson & Tartar, 2006). Notably, a GSII sequence was also found in the nonphotosynthetic dinoflagellate, Oxyrrhis marina, and its affiliation with a diatom plastid-targeted GSII was interpreted as evidence for the presence of a functional secondary plastid in the evolutionary past of this presently nonphotosynthetic dinoflagellate lineage (Slamovits & Keeling, 2008).
The phylogenetic relationships depicted in Fig. 3a are consistent with those reported by Robertson & Tartar (2006) in (i) failing to cluster the red algal and chromalveolate sequences [likely because of limited taxon sampling and/or the lack of red-algal plastid GSII sequences; (Robertson & Tartar, 2006)] and (ii) placing the chloroplast-targeted green algal GSII outside the clade containing all eukaryotic GSII sequences [possibly reflecting a LGT event from a bacterial source; (Robertson et al., 1999)]. However, in contrast to Robertson & Tartar (2006), some of our analyses recovered the monophyly of chromalveolates (Fig. 3a). Interestingly, in the haptophyte, Emiliania huxleyi, we identified two distinct GSII sequences: one that groups with diatom plastid-targeted sequences (this grouping is also supported by the presence of a putative signal peptide, characteristic of sequences targeted to the secondary plastids), and one that affiliates with oomycete cytosolic sequences (Fig. 3a). As E. huxleyi possesses cytosolic GS sequences of type III (e.g. Maurin & Le Gal, 1997), the putative cytosolic GSII sequence in E. huxleyi represents an addition to the haptophyte gene complement, which suggests a rather complex evolutionary history for GS in the chromalveolate lineage.
Because of this unexpected complexity and the recently recognized issue of multiple LGTs (from both endosymbiotic and food sources) and consecutive endosymbiotic replacement events in lineages with secondary plastids (e.g. Frommolt et al., 2008), which are likely to affect phylogenetic inferences, we also performed analyses restricted to lineages possessing primary plastids. In all analyses (i.e. both Bayesian and maximum likelihood; with or without prokaryotic sequences; with additional taxa), Monosiga GSII sequences failed to branch within the Opisthokonta; instead, they consistently branched within the Plantae group (Fig. 3b and Fig. S3). Furthermore, the exclusion of the chromalveolate sequences increased the support for the Monosiga GSII sequences grouping with homologues from photosynthetic lineages (Fig. 3b).
Overall, the current data indicate that the Monosiga GSII genes are of algal origin [note that GSII sequences have not been reported in cyanobacteria (Robertson & Tartar, 2006)], but the exact nature of the algal donor cannot be inferred at this time. The presence of algal-related GSII genes in both Monosiga species, which belong to two early diverged clades within Choanoflagellata (Carr et al., 2008), indicates that the acquisition event took place early in the evolution of this group. This conclusion is consistent with our previous finding of two ascorbate peroxidase and two metacaspase genes that have also been acquired from an algal donor early in the evolutionary history of choanoflagellates (Nedelcu et al., 2008).
Ammonium transporters of the AMT/Rh family have been described in archaea, bacteria, fungi, algae, plants, and invertebrates – but are missing in vertebrates; on the other hand, Rh glycoproteins – abundant in vertebrates, are also found in invertebrates, slime moulds, algae, and some bacteria, but are missing in vascular plants (Huang & Peng, 2005). Nevertheless, the two types of proteins appear to have co-existed for a long time, as both AMT and Rh sequences are found in many lineages, including slime moulds, green algae, oomycetes, and invertebrates (Huang & Peng, 2005).
Our searches in the M. brevicollis genome database retrieved up to five AMT genes (though only three have reliable gene models) and one gene encoding an Rh-like protein; in addition, we also found an AMT sequence in the M. ovata EST database. Interestingly, phylogenetic analyses indicate that the predicted AMT transporters from the two Monosiga species belong to two distant clades (Fig. 4a). The three M. brevicollis predicted AMTs and some animal homologues cluster together within a large clade of AMT sequences from photosynthetic lineages, including members of the Arabidopsis AMT1 family of high-affinity transporters (von Wiren et al., 2000), called here the AMT1 clade. On the other hand, the M. ovata AMT sequence branches with homologues from the amoebozoan, Dictyostelium discoideum, and the excavate, Trypanosoma cruzi, in a distant clade that also contains fungal, oomycetes, green algal and plant sequences, including the well-characterized Arabidopsis AMT2 transporter (i.e. the AMT2 clade) (Fig. 4a).
The separation of plant AMTs in two clades, one of which branches with fungal and bacterial homologues has been previously reported (Gonzalez-Ballester et al., 2004; Ludewig et al., 2007). Surprisingly, although the animal AMT sequences are also split into two groups, neither of them clusters with fungal homologues, which form a distant branch in the AMT2 clade (Fig. 4a). The split of animal AMT sequences in two groups as well as the close relationship between one of the animal AMT groups and algal/plant homologues (Fig. 4a) is consistent with previous analyses of AMT and Rh sequences (Huang & Peng, 2005). An affiliation between the AMT sequences from the M. brevicollis/animal group and those from photosynthetic lineages (including lineages with secondary plastids, such as the diatoms) is supported by an insertion that is shared by sequences in the AMT1 clade, to the exclusion of all other AMT sequences (Fig. 4b).
Notably, the only lineages that appear to posses the two distinct types of AMT transporters are the green plants (i.e. green algae and land plants), Monosiga, and the amoeobozoan, D. discoideum (Fig. 4a). The presence of both AMT1- and AMT2-like sequences in these lineages can, in principle, reflect a duplication event that took place before the divergence of the lineages leading to green plants, Monosiga and Amoebozoa. However, as Choanoflagellata and Amoebozoa – on the one hand, and Viridiplantae (i.e. green plants) – on the other hand, are representative of the two main eukaryotic lineages, the Bikonts and the Unikonts [believed to have diverged very early in eukaryote evolution; (Stechmann & Cavalier-Smith, 2003)], such a scenario will require independent differential losses in many lineages. These include the loss of AMT1-like sequences in excavates, oomycetes and fungi, and the loss of AMT2-like sequences in diatoms and animals. Furthermore, if this were the case, the AMT1-like sequences from D. discoideum and M. brevicollis should cluster together – to the exclusion of Plantae and chromalveolate homologues; instead, D. discoideum AMT1-like sequences branch away from M. brevicollis AMT1 sequences, and affiliate with a specific group of green algal AMT1-like homologues (Fig. 4a).
In this context, LGT events appear to be a more likely explanation for the observed AMT distribution and affiliations. Specifically, the algal-related AMT1 sequences from D. discoideum can be interpreted as AMT additions (from a Chlamydomonas reinhardtii-like alga) to its existing AMT2 complement. Notably, both D. discoideum and C. reinhardtii are soil-dwelling species, and LGT events are thought to be facilitated by the donor and recipient inhabiting the same habitat (Andersson, 2005; Andersson et al., 2006). Furthermore, D. discoideum is known to have acquired several bacterial genes, many of which are related to living in the soil (Eichinger et al., 2005). Likewise, the presence of AMT1-like sequences in diatoms, while their nonphotosynthetic relatives, the oomycetes, only posses AMT2-like sequences (Fig. 4a), can be interpreted as the result of an endosymbiotic transfer event from the photosynthetic red algal endosymbiont followed by the replacement of the resident AMT2 sequence (note that we have identified AMT1-like sequences in the red algae, Cyanidioschyzon merolae (http://merolae.biol.s.u-tokyo.ac.jp/) and Galdieria sulphuraria (http://genomics.msu.edu/galdieria/), but have excluded them from the phylogenetic analysis in Fig. 4a because of their diverged sequences).
A similar scenario can also be envisioned for M. brevicollis: the clustering of the M. brevicollis AMT1-like sequences with algal/plant homologues can indicate an algal origin for the former. As AMT2-like sequences are present in Amoebozoa, fungi, and M. ovata, it is likely that the last common ancestor of the lineages leading to M. brevicollis and M. ovata possessed an AMT2 sequence. However, we were not able to identify an AMT2 homologue in the available genomic sequence of M. brevicollis, which allows for the possibility that the laterally acquired AMT1 sequence replaced the resident AMT2 in the lineage leading to M. brevicollis. Notably, AMT2 sequences are also missing in the C. reinhardtii genome, and it was suggested that in this lineage the role of AMT2 transporters was undertaken by some of the AMT1 proteins (Gonzalez-Ballester et al., 2004). On the other hand, although genomic information from M. ovata is not available, the fact that AMT1-like sequences are present in both M. brevicollis and metazoans indicates that the acquisition event took place before the divergence of the Choanoflagellata and Metazoa, and thus, before the divergence of the two Monosiga lineages. Consequently, both AMT1 and AMT2 sequences would have co-existed in the primitive Monosiga, and the AMT1 would have replaced AMT2 in the lineage leading to M. brevicollis.
When discussing LGTs and their evolutionary significance, several issues are especially relevant: the type of gene (novel or homologous to an endogenous gene); its contribution to the host gene complement (gene addition or functional replacement); number of genes transferred (single gene or multiple genes); the type of product encoded (secondary or primary metabolite); functional integration (single protein or part of a pathway); origin (prokaryotic or eukaryotic); effect on the fitness of the host (neutral or adaptive). Many laterally acquired genes are single genes coding for novel proteins that confer an adaptive advantage to the recipient (e.g. Andersson, 2005; Keeling & Palmer, 2008). However, entire pathways can also be acquired, if the functionally related genes are physically clustered on the genome, such as in prokaryotic operons (e.g. Craig et al., 2008). Although it was initially believed that core genes are not likely to be transferred [because the recipient already possesses functional genes whose products are well integrated in complex metabolic pathways; (Lawrence, 1999; Pal et al., 2005; Merkl, 2006)], genes related to primary metabolic pathways are now known to also be acquired laterally (Omelchenko et al., 2003; Lima et al., 2008, 2009). However, functional replacements of entire primary metabolic pathways are thought to occur only under special circumstances (see below).
Generally, several scenarios can be envisioned to account for functional replacement events. For instance: (i) the foreign and native genes initially co-exist, and because of the resulting functional redundancy, one of the two genes is lost; (ii) the two genes initially co-exist, and if not fully equivalent, the acquired gene can be selectively retained; and (iii) the native gene has been lost in response to a prior change in ecological niche or life-style, but when circumstances change again, a foreign homologue is re-acquired. In the first scenario, the fixation of the foreign gene implies stochastic events, which can be facilitated by a ‘ratchet mechanism’ (Doolittle, 1998). On the other hand, the second scenario entails a selective benefit associated with the replacement, and while such events are theoretically possible, they are more difficult to document (Huang & Gogarten, 2008). Lastly, the third scenario was recently invoked to explain the replacement of the arginine biosynthesis pathway in a group of bacteria (the Xantomonadales), and the re-acquisition of the vitamin B pathway in the parasitic nematode, Heterodera glycines; in both cases, it was proposed that the resident pathway was lost following the switch to a parasitic life-style, and was later re-acquired laterally, when the conditions became restrictive again (Craig et al., 2008; Lima & Menck, 2008). Overall, while the events in the former scenario may be considered selectively neutral (Gogarten & Townsend, 2005), the latter two scenarios call for a selective advantage associated with the retention of the acquired gene, and thus, are adaptive.
The data reported here argue that the M. brevicollis genes coding for GS, GLTS and one family of ammonium transporters are of algal origin. Furthermore, these acquisitions appear to have involved the functional replacement of the resident homologues. As the three genes are not known to be physically clustered in any system, it is most likely that they were acquired independently. Although the independent acquisition and physical integration in the recipient genome of these three genes can be understood as a consequence of stochastic events, it is less probable that chance events were also responsible for the concerted retention of all three genes, at the expense of the native homologues; rather, it is more likely that these three foreign sequences whose products work together in a primary metabolic pathway have been retained because they provided a selective advantage over the endogenous genes. The fact that these genes appear to have been acquired from the same type of donor (i.e. a photosynthetic alga) further supports this suggestion.
Thus, according to the theoretical scenarios described above, the adaptive replacement of the three genes in M. brevicollis could be envisioned following scenario 2 (i.e. the selective retention of the acquired homologue) or 3 (i.e. the prior loss of the native gene followed by the re-acquisition of a foreign homologue under new adaptive pressures). The latter scenario requires two major changes (to account for both the loss and the re-acquisition events) in the ecology and/or life-style of the lineage leading to the extant M. brevicollis. While this possibility cannot be entirely excluded, there is no indication that such changes have occurred in the evolutionary history of Monosiga (all extant Monosiga are aquatic, phagotrophic, free-living species). Consequently, the second scenario, which requires that the foreign homologues provided adaptive advantages over the native copies, is more likely.
What could such advantages be? The acquisition of a homologous sequence is usually seen as creating functional redundancy; however, proteins encoded by members of the same family can have distinct biophysico-chemical and/or kinetic properties, even within the same species. For instance, five cytosolic GS isozymes are present in Arabidopsis thaliana, and their affinity (i.e. KM) for ammonium varies from 10 μm (for the high-affinity GSs) to 2450 μm (for the low-affinity GSs). Thus, it is conceivable that the native choanoflagellate and the acquired algal GS and GLTS sequences differed in their enzymatic properties in such a way that that the acquired algal sequences provided a benefit to its recipient in terms of substrate affinity or specific activity. It should be noted that potential interdomain LGT events (between Eubacteria and Archaea) involving GS and GLTS sequences have also been reported, and are believed to possibly represent cases of adaptive orthologous replacements (see Raymond, 2005 for a discussion).
In contrast to Rh glycoproteins, which are thought to act as passive transporters of NH3, AMTs can specifically and actively (i.e. against a gradient) transport NH4+ or cotransport NH3/H+ (Ludewig, 2006). AMT/Rh transporters in bacteria, plants and animals are known to differ in their ammonium transport capabilities, and it has been suggested that the functional differences between them are likely to reflect evolutionary adaptations to different ammonium gradients and nitrogen requirements (Ludewig, 2006). In addition, AMTs can also differ vastly in their KM, even in the same organism: from 0.5 to 170 μm– in plants, and from 7 to 30 μm– in Chlamydomonas (Gonzalez-Ballester et al., 2004). Furthermore, the differences between bacterial and plant AMT/Rh transporters are believed to possibly be significant in a competitive soil and provide an evolutionary adaptation to the large nitrogen requirements of plants (Ludewig, 2006).
Interestingly, a rather large number (up to five) of AMT1-like transporters are present in M. brevicollis; the existence of a large number of transporters with different but complementary affinities and activities for the substrate in unicellular organisms such as C. reinhardtii (which has eight AMT1 transporters, the largest known number of AMT1 in a species) is thought to reflect an adaptive strategy to allow an efficient uptake under changing environmental conditions (Gonzalez-Ballester et al., 2004). As ammonium is rather low in marine environments, it is conceivable that the acquisition of high-affinity ammonium-transporters in the ancestor of Monosiga could have provided a selective advantage in this environment, and thus could have been selectively retained. Ammonium is also a constant by-product of amino acid catabolism and de-amination reactions, and at high concentration becomes toxic. Unicellular organisms that lack specialized means to accumulate toxic metabolic products (such as the large vacuole in plant cells) need to have very efficient mechanisms to excrete ammonium at high and toxic concentrations, and also re-uptake the passively lost ammonium, when its extracellular concentration becomes low (Gonzalez-Ballester et al., 2004). In this context, it should be noted that in contrast to many unicellular species, adult Monosiga are sessile, and thus, in the absence of motility (which is also true for land plants), these needs might be greatly intensified.
Although at this time we could only speculate on the adaptive benefit(s) (past or present) of these replacements, it is noteworthy that in plants, in addition to their role in nitrogen assimilation, the enzymes involved in nitrogen metabolism are also thought to play an important role in tolerance against water deficiency and possibly salt stress conditions [e.g. (Ramanjulu & Sudhakar, 1997; Debouba et al., 2006)]. For instance, differences in drought tolerance between two mulberry genotypes were correlated, at least in part, with the ability to maintain greater levels of amino acid pools coupled with a more pronounced re-assimilation of toxic ammonia (Ramanjulu & Sudhakar, 1997). Notably, drought and salinity stress also induce oxidative stress, and in Monosiga several stress-related genes, including two ascorbate peroxidases, involved in coping with oxidative stress, are of algal origin as well (Nedelcu et al., 2008). Remarkably, in nonphotosynthetic dynoflagellates, GSII and ascorbate peroxidase genes are also among the sequences thought to have been acquired from their algal endosymbiont and retained after the loss of photosynthesis (Sanchez-Puerta et al., 2007; Slamovits & Keeling, 2008), suggesting that these laterally acquired algal sequences can be generally adaptive in nonphotosynthetic lineages.
Overall, the data presented here indicate that three Monosiga genes coding for proteins centrally involved in the metabolism of nitrogen are of algal origin and have been acquired early in the evolution of the choanoflagellates. The concerted retention of these independently acquired genes whose products work together in the GS/GOGAT pathway suggests that the functional replacement of the endogenous homologues was adaptive. More generally, this study argues that (i) eukaryote-to-eukaryote transfers of entire metabolic pathways are possible in the absence of the physical clustering of the corresponding genes, (ii) adaptive functional replacements of primary metabolic pathways can occur via multiple independent gene transfers and the selective retention of the acquired sequences, and (iii) functional replacements involving eukaryotic genes could have contributed to the evolution and diversification of eukaryotes. Furthermore, by adding to our previous finding of stress-related genes of algal origin in Monosiga, this report underscores the potential contribution of algal genes to the evolution of nonphotosynthetic lineages.
This research was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada to A.M.N. Many of the sequences analyzed in this study were produced by the US Department of Energy Joint Genome Institute (http://www.jgi.doe.gov/) and the Protist EST Program (http://amoebidia.bcm.umontreal.ca/pepdb/) and are provided for use in this publication only.