Sperm limitation and genetic effects on fecundity in the dioecious liverwort Sphaerocarpos texanus

Spatial relationship, male size and genetic interaction were manipulated to determine their effects on fecundity variation in the dioecious ephemeral liverwort Sphaerocarpos texanus Aust. Genetically identical male individuals and genetically identical female individuals were used within inter-male distance and male size experiments. Thus, any treatment effects within these experiments cannot be attributed to genetic effects. For the genetic interaction experiment, three males and three females were mated in a factorial design resulting in nine unique crosses. Each of these nine crosses was replicated eight times. In addition, 182 pairs (13 males and 14 females) were crossed to detect general trends across many pairings. I found that both increasing inter-mate distance and decreasing male size reduce sporophyte production, thus suggesting sperm limitation. One of the nine pairs had very low levels of sporophyte production. The male and female individuals involved in this pairing did not have lower fecundity levels when their successful matings were compared with the other two individuals of similar sex. Of the 182 pairs, 13 were not fecund. This fecundity pattern must have an underlying genetic mechanism since the individuals involved were fecund in other crosses. This novel result provides evidence that genetic interactions may contribute to the low levels of sexual reproduction observed among dioecious bryophytes.