MCLETCHIE, D. NICHOLAS*, GISELA GARCIA-RAMOS, and PHILIP H. CROWLEY. Center for Ecology, Evolution and Behavior and T.H. Morgan School of Biological Sciences, University of Kentucky, Lexington, KY 40506-0225. - A model of local sex-ratio dynamics and spore production in the dioecious liverwort Marchantia inflexa.
In many dioecious bryophyte species, population sex ratios range from
all female to all male. The focal species of the present study, the
liverwort Marchantia inflexa, forms patches on rock surfaces,
and these differ widely in sex ratio at a rainforest field site in
Trinidad. This analysis addresses abundances of male and female M.
inflexa through time within an individual patch. We represent the
life-history of this species using seven different stages
(non-reproductive, asexually reproductive, and sexually reproductive
males, non-reproductive and asexually reproductive females, and
unfertilized and fertilized sexual females) and express their dynamics
using ordinary differential equations. Some of the stages grow by
extending thalli over the substrate and may overgrow each other to
capture space. Our simple representation of dynamics within the patch
failed to stabilize the sex ratio: females gradually eliminated males
at low disturbance frequency, and males eliminated females at high
disturbance frequency. This pattern did not hinge on whether sexual
propagules could germinate within the patch, but asexual reproduction
(via gemmae dispersed within the patch) played an important role. This
suggests that the maintenance of sex in these populations may hinge on
metapopulation structure and dynamics. Though sexual reproduction
appears to be unimportant within patches, spores provide the primary
means of recolonizing patches eliminated by large-scale disturbances.
We found that shortly after the patch was fully occupied, the
production of these wind-dispersed spores was maximized, but spore
production declined thereafter as the sex ratio became increasingly
biased toward one sex or the other. Much additional modeling and
empirical work is needed to enable linkage of within-patch dynamics to
represent the metapopulation.
Key words: bryophytes, clonal organisms, Marchantia inflexa, metapopulations, sex-ratio dynamics