Moss sex: mating systems and the evolution of combined and separate sexes in gametophytes

Abstract

Mosses show considerable breeding system variation, with approximately half of all species having separation of sex function between individuals. Currently there are no studies that have examined the factors selecting for this variation. Although inbreeding depression is considered an important factor influencing the evolution of breeding systems in seed plants, its role in bryophytes is often overlooked. In this thesis I examined the role of inbreeding depression in the maintenance of separate sexes in mosses. First, allozyme electrophoresis was employed to measure selfing and outcrossing rates in five species of moss with combined sexes (monoicy) and five species with separate sexes (dioicy). On average monoicous mosses had lower levels of heterozygote excess than dioicous mosses; this suggests that selfing rates are higher in monoicous species than dioicous species. To test the hypothesis that inbreeding depression would be higher in mosses with separate sexes, the relative performance of self-fertilised and outcrossed sporophytes were compared in a monoicous moss (Funaria hygrometrica) and a dioicous moss (Ceratodon purpureus). There was no significant inbreeding depression in any of the four sporophyte fitness traits measured in the monoicous moss. In the diocious species there was a tendency for increased spore number, capsule weight, seta length and capsule length in the outcrossed sporophytes, and two of the traits showed inbreeding coefficients significantly different from zero. This research suggests that inbreeding depression and selfing rates may be important factors in the evolution and maintenance of combined and separate sexes in mosses. However, other factors such as resource availability and sexual specialisation of the sexes should be investigated further.