One down and thousands to go – dissecting polyploid speciation
The literature on speciation is vast, but there are few undisputed examples for any given model of the process. The exception is polyploid speciation. Because of strong postzygotic reproductive isolation of tetraploid derivatives, the most common class of polyploids, from their diploid progenitors – which comes about because triploid hybrids are highly sterile – the observation of more than one established cytotype is prima-facie evidence of polyploid speciation. Given that estimates of polyploid speciation in angiosperms from a minimum estimate of 2%–4% (Otto & Whitton, 2000) to perhaps as much as 20%–40% (Stebbins, 1938), botanists have thousands of examples of polyploid speciation in flowering plants alone from which to enhance our understanding of this mechanism of diversification. While the literature on polyploidy has exploded in recent years, the emphasis of current polyploidy research largely focuses on genetic and epigenetic consequences of genome duplication. We are now beginning to understand that the impacts of polyploidization on the genome are varied and complex (Osborn et al., 2003), and while such studies inform us about the impacts of genome duplication, they have thus far offered few direct insights into polyploidization as a mechanism of speciation. To understand polyploid speciation, we need to understand how novel cytotypes arise, become established and persist in nature, and this is where the work of Brian Husband on fireweed, on pp. 701–711, is leading the way.
‘It is a matter of time, and probably not that much time, before we can understand the direct contribution of genome duplication in polyploid speciation’
Studies with fireweed
Brian Husband and coworkers’ studies of fireweed (Chamerion angustifolium) provide the most comprehensive set of experiments on the dynamics of cytotype interactions available for any polyploid system. In North America, fireweed includes diploid and tetraploid cytotypes that have generally distinct ranges, with diploids occurring further North and at higher elevations. A broad contact zone exists in the Rocky Mountains, with the cytotypes segregating along the elevational gradient (Husband & Schemske, 1998). In this case, the polyploids are presumed autopolyploids that have not been granted taxonomic status (as is typically the case for autopolyploids), though the cytotypes are distinct biological species, with diploids and tetraploids having largely distinct ranges and hybrids between them being almost completely sterile (their fitness relative to diploids was estimated at 0.09; Burton & Husband, 2000). Husband & Sabara (this issue) uses a number of previous studies to develop a comprehensive estimate of the contributions of a suite of six pre- and post–zygotic reproductive isolating barriers in the contact zone. They apply an intuitive method, described by Coyne & Orr (1989, 1997; see also Ramsey et al., 2003), that considers the timing of action of each barrier and uses the isolation achieved at each step to scale the contributions of later–acting barriers. Using this approach, they estimate that the two fireweed cytoypes achieve more than 99.7% reproductive isolation in nature. Perhaps unexpectedly, most of the isolation is achieved before fertilization, via spatial isolation, flowering time divergence and pollinator fidelity. Together these three barriers account for more than 92% of the observed isolation, so while hybrid sterility provides a strong barrier between the cytotypes, its late action decreases its current importance.
In all studies of reproductive isolation, a key focus is to understand which isolating barriers were critical in the initial stages of speciation. In the case of polyploid speciation, Levin (1975) and others have pointed out that the future for a newly arisen tetraploid in a sea of diploids is not so bright as it will face minority cytotype disadvantage, the likely fate of being swamped to extinction by maladaptive hybridization with diploid progenitors. Thus it has long been recognized that factors which enhance the probability that a newly arisen tetraploid produces tetraploid offspring are likely critical to the establishment of nascent polyploid species. Such factors include spatial (ecological) isolation, increased self-compatibility, shifts in developmental timing or pollinator preferences. Indeed, each of these traits contributes to total isolation in Chamerion angustifolium. In addition, the well documented tendency for recurrent polyploid formation and/or recurrent formation of unreduced gametes in diploids (and perhaps in triploids as well; Ramsey & Schemske, 1998) may contribute to the gamete pool for successful production of polyploid offspring that could help overcome the initial minority cytotype disadvantage.
Reproductive barriers during the establishment of the polyploid cytotype
The difficulty with using established polyploids to study the early stages of establishment is that the barriers that are currently present may have had a variety of origins. First, phenotypic changes that confer reproductive isolation may be the direct consequence of genetic or epigenetic changes that accompanied polyploidization. Second, prezygotic isolation may have been enhanced via reinforcement (selection to avoid maladaptive hybridization). Third, some of the additional isolating barriers may have arisen following polyploid establishment, and represent the incidental by-products of divergence. We must, however, keep in mind that the origin of the polyploid cytotype must have been sympatric or nearly so, and therefore to overcome minority cytotype disadvantage, some barrier other than hybrid sterility likely contributed to initial establishment. This suggests that barriers that have a tendency to arise as an immediate by-product of genome duplication may have been most likely to act in polyploid establishment.
Previous work on the phenotypic consequences of genome duplication (Levin, 1983; Ramsey & Schemske, 2002) provide potential insight into which pre–zygotic barriers are most likely to have been in place early in the speciation process. However, as pointed out by Ramsey & Schemske, many, indeed most, reports of phenotypic correlates of polyploidy are difficult to interpret because they confound divergence that may have occurred subsequent to polyploid establishment with effects that are attributable to polyploidy per se. Nonetheless, based on a review of studies of newly synthesized polyploids, Ramsey & Schemske (2002) find evidence that neopolyploids differ in phenology from their progenitors, with flowering typically initiated later. This difference is in the same direction as observed in fireweed. Pollinator fidelity, the tendency for pollinators to fly within rather than between cytotypes, has previously been noted in Heuchera grossularifolia (Segraves & Thompson, 1999), and represents a significant reproductive barrier in fireweed. No studies of neopolyploids indicate pollinator shifts as a direct consequence of genome duplication, though changes in flower size that could potentially be associated with shifts in pollinator fidelity or that could contribute to shifts in selfing rate or success rate of intercytotype pollination have been noted (Ramsey & Schemske, 2002). Spatial isolation resulting from shifts in habitat tolerances are frequently cited as important in polyploid establishment, yet I am unaware of a single study that compares ecological tolerances of newly synthesized polyploids to their diploid progenitors. Thus, although this spatial isolation is important at present in Chamerion, we have no way to evaluate the likelihood that such a shift occurred early in polyploid establishment. To further our understanding of the early stages of polyploid establishment, it would be ideal to use newly synthesized polyploids of Chamerion to evaluate the shifts that accompany genome duplication. In addition, an understanding of the evolutionary history of the contact zone would allow a better understanding as to whether the dynamics observed reflect early stages in polyploid establishment or secondary contact between cytotypes that originated elsewhere and have diverged in allopatry.
Linking ecological and genetic studies
While the current emphasis of polyploidy research emphasizes genetic and epigenetic shifts, in order to gain a complete understanding of how species arise by polyploidization we must also understand how these entities become established and persist in nature. The chasm between these two fields of endeavor is narrowing: it is now possible to compare gene expression in synthetic and natural polyploids (Adams et al., 2003), and while, thus far, the ecological significance of such expression differences remains elusive, candidate loci for potentially ecologically significant traits are becoming well-characterized. This implies that it is a matter of time, and probably not that much time, before we can understand the direct contribution of genome duplication in polyploid speciation. However in order to fulfil this promise, we will need to have a thorough understanding of the operation of reproductive isolation and the nature of ecological divergence in natural polyploid systems.