What are the fitness and evolutionary consequences of the Y chromosome?

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Adaptive explanations of the existence of male (Y) chromosomes have been coupled to increased individual fitness with reduced inbreeding and/or stronger sexual selection. Because the outcome of the two processes is increased fitness, it has, thus far, proved difficult to discriminate among them empirically. Experimental evidence to test the relative importance of Y chromosomes in these two important evolutionary processes is rare (if not absent, in a recent literature search) because it is also difficult to find dioecious organisms that can produce male phenotypes and gametes without also possessing the Y chromosome. Cannabis sativa is a dioecious plant with sex determined by an XY system; however, sex expression in this species is sensitive to environmental context. Thus, sex chromosomes and sex expression in cannabis can be decoupled: 1) plants can possess XY chromosome complements but produce either male or female flowers, or 2) they can possess XX chromosome complements but produce either male or female flowers. This plant provides an excellent platform to test basic hypotheses about the role of the Y chromosome in sexual differentiation and expression, sexual selection and inbreeding avoidance. The long-term objective of my research program is to discover principles that drive evolution as a consequence of demography, especially mating processes. We are in a unique position to test basic questions about chromosome complement and mating using a plant with flexible sex expression. Insights gleaned from this plant are transferrable to other dioecious plants (including Asparagus, Rumex, and hops). My research program will study the reproductive biology of cannabis by exploring: how genetic and environmental factors influence a) sex-specific heritability of quantitative traits, and b) shifts in sex expression; the consequences of flexible sex expression for: a) life history and fecundity, and b) the genetic composition of gametes and embryos; and the relative importance of Y chromosomes in reducing inbreeding or increasing sexual selection.This research program will allow us to finally separate the process of sex from sex chromosomes while still maintaining a dioecious system, and allow us to assess the role of the Y chromosome in mating and sexual selection. Furthermore, this research will train students in a diversity of skillsets, including ecological and population genetics, and protein biochemistry.