The comparative biology of moss sexual systems

Abstract

Separate sexes is very common in animals (95%), and uncommon in flowering plants (6%) suggesting that different life-history strategies experience different selection pressures on sexual system. Mosses remain relatively unstudied, but it may be expected that there would be similar patterns to those found in the angiosperms. However, between 60 and 66% of mosses exhibit separate sexes. I used the comparative method to search for life-history correlates of sexual system in mosses, and to determine if once a sexual system has evolved in a lineage, if it is possible for there to be a reversion. Separate sexes in mosses was associated with asexual reproduction, low chromosome numbers, and large stem size, while combined sexes was associated with high chromosome numbers, and small plant size. In addition, there were significant transitions both to and from combined and separate sexes, suggesting that these transitions are not irreversible, and are likely associated with different selection pressures. Finally, I examined the effect that sexual system has on the success of the lineage, by comparing species numbers that are in separate sex clades with the numbers of species that are in combined sex clades. There was no significant difference in species number, suggesting that sexual system in mosses does not lead to greater extinction or speciation rates. The insight gained by this thesis will serve to extend current models of sexual system evolution by adding information about the evolution of sexual system in taxa with a gametophyte dominant life-cycle.