On the origin and evolution of apomixis in Boechera

Abstract

The genetic mechanisms causing seed development by gametophytic apomixis in plants are predominantly unknown. As apomixis is consistently associated with hybridity and polyploidy, these confounding factors may either (a) be the underlying mechanism for the expression of apomixis, or (b) obscure the genetic factors which cause apomixis. To distinguish between these hypotheses, we analyzed the population genetic patterns of diploid and triploid apomictic lineages and their sexual progenitors in the genus Boechera (Brassicaceae). We find that while triploid apomixis is associated with hybridization, the majority of diploid apomictic lineages are likely the product of intra-specific crosses. We then show that these diploid apomicts are more likely to sire triploid apomictic lineages than conspecific sexuals. Combined with flow cytometric seed screen phenotyping for male and female components of apomixis, our analyses demonstrate that hybridization is an indirect correlate of apomixis in Boechera.

Fig 1

Patterns of heterozygosity indicate differential origins of 2x and 3x apomixis. Genotypes for eight offspring were calculated for each of 1000 simulated crosses among diploid populations of B. pendulocarpa (Intra sim.) and between B. pendulocarpa and the other diploid species (Hyb. sim). Observed heterozygosity was calculated as the proportion of heterozygous markers in each simulated population (intra=blue line, hybrid= pink line) and for the actual genotypes in the populations (black line). The kernel density distribution is plotted against the observed heterozygosity for both the 3xApo accessions (a) and the 2xApo accessions (b). The mean heterozygosity of sexual diploids is labeled in each plot. P-values from pairwise t-test report the significance of the difference between the mean simulated and real data.

Fig 2

The position of apomictic triploid hybrids relative to parental species (split by reproductive mode) in genetic space. Principle components reveal that apomictic diploid lineages are more likely to sire triploid lineages than diploid sexual conspecific lineages in two comparisons: B. stricta X B. retrofracta (upper panel), B. stricta X B. pendulocarpa (lower panel) PCA scores for the 1^st two axes and 95% confidence ellipses are plotted. As in fig 1, triploids are pink and diploid apomicts are blue.

Fig 3

The relative presence of male apomeiosis in 27 diploid female apomictic Boechera lines. The male and female components of apomixis are usually both present in an individual that produces ovules via apomeiosis. Production of reduced pollen and unreduced ovules is limited to three individuals. Mean (+/−SE) %pollen type for each individual is presented (384 seeds/individual) as the solid (unreduced) and dashed (reduced) lines. Individuals are classified as balanced or unbalanced apomicts by the background color. Note: these families are not represented in the population genetic analysis. See Aliyu et al. (2010) for information on these lines.

Fig 4

A conceptual model for the generation of apomixis in Boechera. The five genotypes that have led to the current patterns of apomixis are portrayed in the boxes. The text at the top of the boxes describes 1) the stable seed ploidy following a generation of self pollinating and 2) the female reproductive mode. The male and female symbols highlight the expected ploidy of pollen and embryo sacs respectively. Unreduced gametes have grey fill. Arrows connecting gametes to boxes indicate the contribution of that gamete for each formative mating. The four steps, which are described in the text, are highlighted (i–iv). For example, in step (iii), fertilization of a reduced ovule (from a line which only produces unreduced pollen) by reduced pollen (from the female apomeiosis-only individual) would produce a seed with a 2:3C embryo:endosperm ratio, but the genetic factors which cause the production of unreduced gametes are also transferred. Therefore, the offspring will produce seed with 2:6C ratio when the endosperm is self pollinated.