Different molecular changes underlie the same phenotypic transition: origins and consequences of independent shifts to homostyly within species

The repeated transition from outcrossing to selfing is a key topic in evolutionary biology. However, the molecular basis of such shifts has been rarely examined due to lack of knowledge of the genes controlling these transitions. A classic example of mating system transition is the repeated shift from heterostyly to homostyly. Occurring in 28 angiosperm families, heterostyly is characterized by the reciprocal position of male and female sexual organs in two (or three) distinct, usually self-incompatible floral morphs. Conversely, homostyly is characterized by a single, self-compatible floral morph with reduced separation of male and female organs, facilitating selfing. Here, we investigate the origins of homostyly in Primula vulgaris and its microevolutionary consequences by integrating surveys of the frequency of homostyles in natural populations, DNA sequence analyses of the gene controlling the position of female sexual organs (CYPᵀ), and microsatellite genotyping of both progeny arrays and natural populations characterized by varying frequencies of homostyles. As expected, we found that homostyles displace short-styled individuals, but long-style morphs are maintained at low frequencies within populations. We also demonstrated that homostyles repeatedly evolved from short-styled individuals in association with different types of loss-of-function mutations in CYPᵀ. Additionally, homostyly triggers a shift to selfing, promoting increased inbreeding within and genetic differentiation among populations. Our results elucidate the causes and consequences of repeated transitions to homostyly within species, enabling a likely explanation for the fact that homostyly has not become fixed in P. vulgaris. This study represents a benchmark for future analyses of losses of heterostyly.

Different molecular changes underlie the same phenotypic transition: origins and consequences of independent shifts to homostyly within species

The molecular basis of phenotypic convergence, a key topic in evolutionary biology and ecology, has been investigated especially between species. However, it remains unclear whether mutations in the same or different positions of the same gene, or in different genes underlie phenotypic convergence within species. A classic example of convergence is the transition from outcrossing to selfing in plants, illustrated by the repeated shift from heterostyly to homostyly. Heterostyly is characterized by the reciprocal position of male and female sexual organs in two (or three) distinct, incompatible floral morphs, while homostyly is characterized by a single, self-compatible floral morph. Primula has long served as the prime model for studies of heterostyly and homostyly. Here, we elucidate the phenotypic and molecular origins of homostyly in P. vulgaris and its microevolutionary consequences by integrating microsatellite analyses of both progeny arrays and natural populations characterized by varying frequencies of homostyles with DNA sequence analyses of the gene controlling the position of female sexual organs (CYPᵀ). We found that: homostyles evolved repeatedly from short-styled individuals in association with different types of loss-of-function mutations in CYPᵀ and, consequently, short-styled individuals occur at lower frequencies than long-styled individuals across populations with all three morphs; the shift to homostyly promotes a shift to selfing; and intra-population frequency of homostyles is positively correlated with selfing rate and inbreeding level, increasing genetic differentiation among populations. These results elucidate the connections between the genotypic and phenotypic levels of convergence and the effects of contrasting floral morphologies on reproductive strategies.

The influence of floral morph ratios and low plant density on mating and fertility in a tristylous colonizing species

Sexual reproduction in heterostylous populations may be vulnerable to demographic conditions because of the small number of mating types in populations. Here, we investigate mating and fertility under natural and experimental conditions in tristylous Lythrum salicaria L., an invasive species that exhibits a wide range of floral morph ratios and demographic contexts. We grew 147 open-pollinated seed families from six populations with different morph structures to estimate intermorph mating (d). In a field experiment, we used progeny ratios from 47 spatially isolated individuals to estimate d, and measured the intensity of pollen limitation experienced by the morphs. The M- and S-morphs experienced high rates of d, regardless of population size or morph ratio. Estimates for the L-morph revealed low levels of intramorph mating in three dimorphic and two trimorphic populations, but near complete intramorph mating in a monomorphic population. Despite high levels of intermorph mating in the field experiment, the morphs experienced significant pollen limitation of fruit and seed set, but this did not differ in intensity among the morphs. Our field experiment demonstrates that although plant isolation was associated with pollen limitation of seed set, “long-distance” bee-mediated pollen flow served to maintain intermorph mating. Tristyly in L. salicaria is remarkably robust to the demographic variation associated with colonization.

Variation in floral morph ratios in tristylous Oxalis squamata (Oxalidaceae): an Andean alpine endemic

Negative frequency-dependent selection resulting from disassortative mating should result in equal morph ratios in tristylous populations at equilibrium. However, surveys of morph-frequency variation in tristylous species commonly report deviations from equality. Here, we report variation in morph ratios in Oxalis squamata Zucc., an endemic of the Andean region of Chile and Argentina. Absence of clonal propagation in this species allows unambiguous estimates of the morph ratio of genets. We sampled floral morph ratios in 20 populations occurring in Central Chile and investigated the relation between morph evenness in populations and their size. All populations of O. squamata were tristylous but with significant heterogeneity among populations in morph ratios. Although small populations exhibited a greater variance in morph evenness, biased ratios were also evident in several large populations. We found no evidence of morph loss or a consistent bias in morph frequencies as reported in some tristylous species. Biased morph frequencies in large populations probably arise from episodic sexual recruitment following disturbance and a slow approach to equilibrium.