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- Supporting Information
Nuclear DNA content, or genome size (GS), varies > 1000-fold among angiosperms (Muñoz Diez et al., 2012). Beyond its structural impacts on the genome, DNA content variation may have ecological and evolutionary significance (Biemont, 2008), because GS correlates with a range of phenotypes, including flowering time, flower size, seed mass, leaf size and photosynthetic rate (e.g. Meagher & Vassiliadis, 2005; Beaulieu et al., 2007a,b, 2008). Moreover, several studies have reported within-species correlations between GS and ecological variables, such as altitude, latitude and temperature (Knight et al., 2005). These phenotypic and ecological correlates suggest that GS affects the properties of species, such as regional abundances (Herben et al., 2012), colonization rates and invasiveness (Lavergne et al., 2010).
It thus seems likely that GS is shaped by natural selection, but nonselective alternatives have also been proposed to explain GS variation. For example, the skewed distributions of eukaryotic GSs can be explained by a purely mechanistic model in which GS evolves stochastically at a rate proportional to size (Oliver et al., 2007). In vertebrates, recombination rates, rather than natural selection, seem to drive changes in GS (Nam & Ellegren, 2012). It has also been argued that correlation between GS and population or biological parameters may be blurred by the phylogenetic signal. After correcting for this signal, Whitney et al. (2010) reported that GS in angiosperms does not correlate with effective population size (Ne). Because the efficacy of selection is expected to scale with Ne, the lack of relationship between GS and Ne may indicate that selection has had little impact on the broad-scale evolution of GS in higher plants.
Population-level analyses offer the best opportunity to infer selection on GS (Petrov, 2001), but most analyses of the evolutionary processes acting on GS have taken place on an interspecies scale. Within the plant kingdom, GS has perhaps been best studied within the genus Zea. The genus includes the species Zea mays, which is typified by the domesticated subspecies maize (Zea mays ssp. mays) and two prominent wild subspecies – Zea mays ssp. mexicana and Zea mays ssp. parviglumis. These two wild taxa, both of which are native to Mexico and collectively referred to as ‘teosinte’, are geographically and ecologically distinct. Subspecies mexicana is restricted primarily to the highlands in the states of Chihuahua, Durango and Puebla, as well as the Central Plateau that surrounds Mexico City (Iltis & Doebley, 1980; Fukunaga et al., 2005). At an average elevation of 2135 m (Hufford et al., 2012), mexicana exhibits adaptation to variable temperature conditions and high altitudes. By contrast, ssp. parviglumis is found at lower elevations in the Balsas River Basin and the state of Jalisco, tropical regions with relatively stable temperature regimes and lower average altitudes (1095 m; Hufford et al., 2012). The one location in which the two teosinte taxa overlap is the Balsas River Basin (Fukunaga et al., 2005), which is the presumed location of the domestication of maize from ssp. parviglumis c. 9000 yr ago (Matsuoka et al., 2002; van Heerwaarden et al., 2011).
Although there is still debate as to whether GS commonly varies within species (Knight et al., 2005; Biemont, 2008), there is little doubt that GS varies within Zea mays sensu lato. Among cultivated accessions, which include both open-pollinated landraces and inbred lines, GS varies by at least 30% (Muñoz Diez et al., 2012). This variation exhibits inconsistent patterns with geography. In maize, for example, GS and indirect measures of GS (i.e. the number of chromosomal knobs and C-bands) typically decrease with increasing latitude (Rayburn et al., 1985; but see Rayburn & Auger, 1990), but may be less consistent as a function of altitude (Bennett & Smith, 1976; Rayburn & Auger, 1990). Fewer studies have examined correlates between GS and ecological variables in ssp. mexicana and ssp. parviglumis, but at least one study has found a positive correlation between GS and altitude (Laurie & Bennett, 1985). To the extent that the pattern holds, a negative correlation between GS and altitude/latitude may reflect the need for rapid growth and early flowering in the shorter growing seasons typical of cooler regions (Rayburn et al., 1994; Poggio et al., 1998).
Although it is clear that GS varies within and among the subspecies of Zea mays, there has not yet been a systematic survey of GS variation among taxa and populations. Here, we investigate GS and ecological correlates in large samples of both cultivated landraces of maize and in annual Mexican teosintes. In doing so, we have overcome some of the common limitations of previous studies. The first of these is the sampling; most previous surveys of Zea have assessed GS from c. 20 total isolates. Here, we survey 22 maize landraces and 21 populations of annual teosinte, assessing GS variation from five individuals within each landrace and population. A second limitation is that the geographic scale of sampling has often been so broad that the effects of environmental factors have been blurred. Our sampling along parallel altitudinal gradients is designed specifically to reduce this limitation. Finally, environmental information has been limited mostly to altitude or latitude, without consideration of additional ecological variables that may impact more directly on GS, such as temperature and rainfall.
With GS and environmental data for multiple taxa and specific locations, we address three sets of questions. First, what is the distribution of GS among taxa, populations and plants? Is there convincing evidence that GS varies within taxa in a consistent fashion? Second, does GS correlate with bioclimatic variables? If so, are the correlates consistent across parallel gradients and also among taxa, suggesting a clear relationship between GS and environment? Or might relationships vary between the domesticate and its wild ancestors, as suggested by previous studies? Finally, what does this information reveal about the potential evolutionary and ecological forces that act on GS? Ultimately, we seek to determine whether variation in GS is shaped by, and limited by, ecological and evolutionary factors.