Ecological differentiation facilitates fine‐scale coexistence of sexual and asexual Boechera

Abstract

Premise of the study: Ecological differentiation (ED) between sexual and asexual organisms may permit the maintenance of reproductive polymorphism. Several studies of sexual/asexual ED in plants have shown that the geographic ranges of asexuals extend beyond those of sexuals, often in areas of higher latitude or elevation. But very little is known about ED at fine scales, wherein coexistence of sexuals and asexuals may be permitted by differential niche occupation. Methods: We used 149 populations of sexual and apomictic lineages in the genus Boechera (rock cress) collected across a portion of this mustard’s vast range. We characterized reproductive mode, ploidy, and species identity or hybrid parentage of each individual, and then used a multi-pronged statistical approach to 1) identify ED between sexuals and asexuals; 2) investigate the impacts of two confounding factors, polyploidy and hybridization, on ED; and 3) determine the environmental variables underlying ED. Key results: We found that sexuals and asexuals are significantly ecologically differentiated across the landscape, despite fine-scale interdigitation of these two reproductive forms. Asexual reproduction was strongly associated with greater disturbance, reduced slope, and greater environmental variability. Although ploidy had little effect on the patterns observed, hybridization has a unique impact on the relationships between asexual reproduction and specific environmental variables. Conclusions: Ecological differentiation along the axes of disturbance, slope, and climatic variability, as well as the effects of heterozygosity, may contribute to the maintenance of sexuality and asexuality across the landscape, ultimately impacting the establishment and spread of asexual lineages.

Two cytotype niche shifts are of different magnitude in Solidago gigantea

“…The MANOVA workflow (Rushworth et al, 2018;Hadle et al, 2019) involved a comparison of cytotype multivariate and univariate means. A discriminant function analysis was performed with the 'adegenet' package to identify variables best discriminating the two cytotypes (Jombart, 2008).…”

Identifying the fitness consequences of sex in complex natural environments

“…These asexuals may be polyploid or diploid, enabling separate assessment of the effects of polyploidy and asexuality (Kantama et al 2007;Beck et al 2012;Mau et al 2015). Furthermore, lineages that reproduce asexually are derived from outcrossing events, which may be either interspecific or intraspecific (Li et al 2017;Rushworth et al 2018), allowing separable examination of hybridity and asexuality. Finally, similar to other asexual systems (e.g., Lutes et al 2010;Fradin et al 2017), asexuals are highly heterozygous, whereas sexual Boechera are highly self-fertilizing and thus highly homozygous (Roy 1995;Song et al 2006;Li et al 2017).…”

“…Boechera retrofracta (Brassicaceae) is a short-lived perennial wildflower native to western North America. Multiple Boechera species are highly self-fertilizing (Hamilton and Mitchell-Olds 1994;Roy 1995;Song et al 2006), and B. retrofracta's high microsatellite homozygosity (mean homozygosity = 0.94; Rushworth et al 2018) and small flowers that self-pollinate before anthesis (C. Rushworth, pers. obs.)…”

The evolution of sex is tempered by costly hybridization inBoechera(rock cress)

“…Boechera, and co-occurs with hybridization or with outcrossing among divergent 159 intraspecific populations (Rushworth et al, 2018). The causative relationship between 160 hybridization and asexuality is unknown, although apomixis may be a result of metabolic 161 dysregulation caused by hybridization (Carman, 1997;Sharbel et al, 2010).…”

“…lineages (Rushworth et al, 2018). In 2012, one line was selected from 11 populations of 190 each species for use as crossing parents ( Figure 1, Table S1 in Supplementary 191 Material).…”