Morphometric analysis of the North American creosote bush (Larrea tridentata, Zygophyllaceae) and the microspatial distribution of its chromosome races

Abstract

Polyploidy is a major mechanism of chromosome evolution and speciation in flowering plants. Delineation of polyploid populations as species or subspecies is complicated because of the uncertainties of distinguishing closely related diploids and polyploids in field conditions. Here we evaluate the practical identification of polyploids-using geographic distributions and morphological features-in the North American creosote bush (Larrea tridentata, Zygophyllaceae). Regarded as a classical autopolyploid complex, L. tridentata comprises diploids, tetraploids, and hexaploids distributed throughout the Chihuahuan, Sonoran, and Mojave Deserts. Transect sampling on cytotype range boundaries revealed strong spatial structuring of the chromosome races, and infrequent sympatry, over small spatial scales (\50 km). Inter-cytotype hybrids were rare, with putative triploids and pentaploids comprising \1 % of the sampled plants (N = 1,197). In morphometric analyses of stem, flower, and leaf characteristics (N = 14 traits), we found significant ploidy effects in several cytotype comparisons;reproductive structures and foliage characteristics were particularly discriminatory. Significant transect and ploidy 9 transect effects for most traits suggest, however, that environmental and genic factors influence plant architecture and morphology. Nonetheless, discriminant function analysis with a combined morphometric data set correctly assigned 68.9 % of plants to ploidal level. Pollen diameters increased significantly with ploidal level, providing another potentially informative trait for comparisons of reproductive plants. Taken together, the spatial distribution and morphometric data presented here suggest that the majority of L. tridentata plants could, in principle, be assigned to cytotype in the field. However, because of potential misclassifications, we suggest recognition of the L. tridentata cytotypes as subspecies.

The role of polyploidy in shaping morphological diversity in natural populations ofPhlox amabilis

“…Autopolyploids, or polyploids that form from parents of the same species, have been understudied (Soltis et al, 2007, 2014) despite recent research documenting extensive cytotypic variation within many species, often with complex geographic partitioning and multiple ploidies (Lafuma et al, 2003; Suda et al, 2007; Halverson et al, 2008; Cires et al, 2010; Laport et al, 2012; McAllister et al, 2015; Etterson et al, 2016). One of the effects of polyploidy in plants is morphological change, most notably in cell size (e.g., Otto and Whitton, 2000; Beaulieu et al, 2008), but also in other quantitative traits (e.g., Ståhlberg, 2009; Kim et al, 2012; Laport and Ramsey, 2015). Many of the effects of polyploidy are genomic (Parisod et al, 2010; Gallagher et al, 2016), but understanding the association between polyploidy and morphology is important because the morphological differences between ploidies can have evolutionary and ecological consequences (Li et al, 1996; Segraves and Thompson, 1999; Husband and Schemske, 2000; Maherali et al, 2009; Khazaei et al, 2010; Zozomová‐Lihová et al, 2015; Segraves and Anneberg, 2016).…”

“…Many of the effects of polyploidy are genomic (Parisod et al, 2010; Gallagher et al, 2016), but understanding the association between polyploidy and morphology is important because the morphological differences between ploidies can have evolutionary and ecological consequences (Li et al, 1996; Segraves and Thompson, 1999; Husband and Schemske, 2000; Maherali et al, 2009; Khazaei et al, 2010; Zozomová‐Lihová et al, 2015; Segraves and Anneberg, 2016). Morphological data can also help resolve taxonomic issues in autopolyploid systems (reviewed by Soltis et al, 2007; Laport and Ramsey, 2015; e.g., Judd et al, 2007; Mráz et al, 2011; Pettigrew et al, 2012; Sosa and Dematteis, 2014). For these reasons, studies are needed to advance our understanding of the effects of polyploidy on morphological change of both vegetative and reproductive traits across the distribution of species, and in species with multiple ploidies (Balao et al, 2011; Richardson and Hanks, 2011; Sosa et al, 2012; Laport and Ramsey, 2015).…”

“…Morphological data can also help resolve taxonomic issues in autopolyploid systems (reviewed by Soltis et al, 2007; Laport and Ramsey, 2015; e.g., Judd et al, 2007; Mráz et al, 2011; Pettigrew et al, 2012; Sosa and Dematteis, 2014). For these reasons, studies are needed to advance our understanding of the effects of polyploidy on morphological change of both vegetative and reproductive traits across the distribution of species, and in species with multiple ploidies (Balao et al, 2011; Richardson and Hanks, 2011; Sosa et al, 2012; Laport and Ramsey, 2015). We explore here whether the signature of polyploidy can be detected in the overall phenotype of a species across natural populations when there are multiple ploidies and geographic complexity in cytotype distribution.…”

“…Because cells tend to be larger in polyploids, it follows that metrics associated with cell density or frequency per unit area are negatively correlated with polyploidy, such as stomata per unit area (Levin, 2002; Chen et al, 2009; Maherali et al, 2009; Khazaei et al, 2010). More complex traits such as leaf and flower dimensions, as well as overall habit, can also increase in polyploids (e.g., Husband and Schemske, 2000; Sugiyama, 2005; Kennedy et al, 2006; Vamosi et al, 2007; Chen et al, 2009; Ståhlberg, 2009; Li et al, 2010; Balao et al, 2011; Kim et al, 2012; Laport and Ramsey, 2015; Etterson et al, 2016). Many different kinds of quantitative morphological traits can vary with polyploidy, and, although exceptions exist (e.g., Tate and Simpson, 2004; Tunbridge et al, 2011), the association is generally positive: as ploidy increases, there is a corresponding increase in the size of cells and structures.…”

“…In Larrea tridentata (DC.) Coville a number of traits differed among three ploidies at specific sites where they come into contact (Laport and Ramsey, 2015). However, diploids and hexaploids were not always at the extremes, and tetraploids were not always intermediate, suggesting that the relationship between polyploidy and trait size is not always linear.…”