Patterns of molecular evolution in dioecious and non-dioecious Silene

Area of Science:

• Evolutionary biology
• Plant reproductive strategies
• Genomics

Background:

• Dioecy (separate sexes) is a rare breeding system in flowering plants, potentially leading to extinction.
• Factors like reduced dispersal and sexual selection costs may negatively impact dioecious species' long-term survival.
• Dioecy is hypothesized to decrease effective population size (Ne), reducing selection efficacy and potentially driving positive selection on specific genes.

Purpose of the Study:

• To investigate the evolutionary consequences of dioecy on genome evolution in the Silene genus.
• To disentangle the relative contributions of reduced purifying selection and positive selection in dioecious species.
• To compare sequence evolution between dioecious and non-dioecious species within Silene, where dioecy has evolved multiple times.

Main Methods:

• Comparative genomic analysis of sequence evolution.
• Focus on the Silene genus (Caryophyllaceae), with multiple independent origins of dioecy.
• Examination of species in section Melandrium (older dioecy) and section Otites (recent dioecy).

Main Results:

• In section Melandrium (older dioecy), a global reduction in purifying selection was observed, alongside evidence of positive selection on some male-biased genes.
• In section Otites (recent dioecy), no significant differences in selection patterns were found between dioecious and non-dioecious species.
• Results suggest that the genomic footprints of dioecy may take time to accumulate.

Conclusions:

• Findings support the hypothesis that dioecy might be an evolutionary dead end for flowering plants.
• Reduced effective population size (Ne) is a plausible explanation, though not the only one.
• Dioecious plants experience contrasting evolutionary forces on their genomes, with a time lag before these effects become apparent.