Doctoral Dissertations

Date of Award

5-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Ecology and Evolutionary Biology

Major Professor

Susan E. Riechert

Committee Members

Dvid K. Smith, Gary McCracken, Louis Gross

Abstract

I investigated the changes in sex ratio from germination to senescence in the dioecious liverwort, Sphaerocarpos texanus Aust. The mechanisms affecting these changes and the reproductive consequences of sex ratio are reported. Sphaerocarpos texanus is a winter ephemeral and disperses its spores as tetrads, each consisting of two males and two females. In the field, S. texanus can be found in pure male, pure female and mixed sex clumps. In the laboratory I examined the sex ratio at germination and proportions and sizes of the three clump types at the early and late gametophytic stages. I also examined the reproductive success of pure female and mixed sex clumps, and monitored the growth and survival of individuals in 20 plots over the four month growing season. I also completed a field manipulation experiment (different light conditions) to detect growth differences between males and females. Finally, in a series of laboratory experiments, I addressed specific questions raised by the field results. Two experiments explored the relationship between males and females within mixed sex clumps. In two reproductive experiments I examined the effects of male size and intermate distance on sporophyte production. Because I noted a lack of sporophytes in some mixed sex clumps, I investigated the possibility that some genetic combinations of males and females have higher reproductive output than other genetic combinations. I found both from the field census and the germination experiment that the most abundant clump type was pure female followed by mixed sex clumps. Pure male clumps were least abundant The relative proportion of males further decreased through time during the life cycle. Thus while the female bias begins at or before germination, it continues and becomes more pronounced. In the monitored plots, the most abundant clump type was mixed sex followed by pure female. Overall there was an increasing female bias. The increase in the female bias during the season was a consequence of high male mortality, which appears to result from the action of rain having a disproportionate effect on the smaller pure male clumps. The potential for pure male clumps to join other sex clumps and thus change clump affinity did not contribute to the large decrease of pure male clumps, nor did herbivory. Field studies showed that while the male part of mixed sex clumps were smaller than pure male clumps, males in mixed sex clumps may gain a survival benefit from growing with a female. However, females are competitively superior to males and both sexes are inhibited by leachates of the other sex. Males and females do not appear to exhibit different growth responses to various field manipulations. From the field work, females exhibited greater fecundity in mixed sex clumps than those in pure sex clumps. Reduced sporophyte numbers can be accounted for by sperm limitation due to increasing intermate distance. Smaller male size may also contribute to reduced sporophyte number. The failure of some mixed sex clumps to produce sporophytes may result from the infertility observed in some genetic crosses of S texanus. This infertility must have an underlying genetic mechanism since the individuals involved were fertile in other crosses. Any infertility caused by nuclear genes will not prevent inbreeding within spore tetrads. This pattern of infertility (between pairs of otherwise fertile mates) is unknown for dioecious organisms.

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