Abstract Detail

Annals of Botany Lecture

BARRETT , SPENCER C H [1].

Evoluton of sex ratios and gender strategies in flowering plants.

Gender strategies involve three fundamental sex phenotypes – female, male and hermaphrodite. Their frequencies in plant populations typically define plant sexual systems. Here, I present the results of our recent studies of sex ratios and gender strategies in flowering plants using the tools of comparative biology, ecological genetics and genomics. A large-scale comparative analysis of the ecological and genetic correlates of sex-ratio variation in dioecious angiosperms revealed that male-biased sex ratios were twice as common as female-biased ratios. Male bias was associated with long-lived growth forms, biotic seed dispersal and fleshy fruits. In contast, female bias was associated with abiotic pollen dispersal and sex chromosomes.   Studies of the mechanisms governing female-biased sex ratios in Rumex (Polygonaceae), a genus of wind-pollinated herbs with heteromorphic sex chromosomes (females XX, males XY or XYY), demonstrated that females located in close proximity to males produced more strongly female-biased sex ratios compared to more isolated females. This suggests that male proximity influences progeny sex ratios by affecting the size of pollen loads and the strength of gametophytic competition. We propose that the poor performance of male-determining gametophytes is associated with Y-chromosome degeneration as a result of suppressed recombination and the accumulation of deleterious genes on the Y chromosome. Our recent investigations of sex chromosome evolution in Rumex hastatulus provide support for this hypothesis by demonstrating the early stages of degeneration of Y chromosomes involving reduced gene expression and gene loss.   Studies of gender strategies and sex-ratio variation in the insect-pollinated, clonal aquatic Sagittaria latifolia (Alismataceae) have revealed near continuous variation in sex phenotype frequencies ranging from monoecy through subdioecy (females, males and hermpahrodites) to dioecy. An increase in hermaphrodites and a decrease in females characterizes the northern range limit of dioecious populations. Using genetic markers we demonstrate that subdioecious populations have arisen by two distinct genetic pathways at range limits. The striking sexual diversity in S. latifolia indicates that sex expression can be largely a quantitative trait and warns against forcing species into typological sexual system categories, as commonly practised by many taxonomists.

Related Links:
Barrett lab web page

1 - University Of Toronto, Department Of Ecology & Evolutionary Biology, 25 WILLCOCKS ST, TORONTO, ON, M5S 3B2, Canada

Keywords:
gender strategies
sex ratios
sexual systems
dioecy
sex chromosomes.

Presentation Type: Special Presentation
Session: S2
Location: Summit/Boise Centre
Date: Monday, July 28th, 2014
Time: 11:00 AM
Number: S2001
Abstract ID:79
Candidate for Awards:None