IS DECREASED GERMINATION FRACTION ASSOCIATED WITH RISK OF SIBLING COMPETITION

Seed dormancy is hypothesized to be a risk-spreading strategy that max imizes plant fitness. By spreading germination time of offspring among years, plants may not only vary the environment to which their offspr ing are exposed, but may also reduce potential fitness losses due to s ib competition. The sibling competition hypothesis predicts that large families whose offspring are more likely to experience sibling compet ition should stagger germination over several seasons (i.e. have great er dormancy) to a greater extent than small families. This should be o bserved as a lower initial probability of germination for seeds from l arge families than those from small families. We examined the relation ship between family size and germination behavior in the desert mustar d Lesquerella fendleri. In a single population in central New Mexico, USA, we estimated family size and germination fraction of 189 randomly selected maternal plants. Although a median test was not supportive, the randomization test showed marginal support for the sibling competi tion hypothesis. An additional study of twelve large and twelve small families in each of three other populations also showed a trend (signi ficant in one population) for larger families to have lower germinatio n fractions than small families. These results suggest a degree of sup port for the hypothesis, given that so many selective forces interact to generate patterns of seed dormancy. Thus, we have not ruled out the hypothesis that large families reduce the likelihood of sib competiti on in good years by producing seed crops with initially low germinatio n. Sibling competition as well as environmental heterogeneity may have influenced the evolution of seed dormancy in this system.