CONTRASTING VARIATION WITHIN AND COVARIATION BETWEEN GENDER-RELATED TRAITS IN AUTOGAMOUS VERSUS OUTCROSSING SPECIES - ALTERNATIVE EVOLUTIONARY PREDICTIONS

We present several predictions concerning the expression of genetic va riation in, and covariation among, gender-related traits in perfect-fl owered plant taxa with different breeding systems. We start with the i nference that the pollen:ovule (P/O) ratio in obligately autogamous sp ecies (in which the ovules in a flower are fertilized only by the poll en it produces) should be under much stronger stabilizing selection th an in outcrossing taxa. Consequently, we predict that obligately autog amous taxa should exhibit lower genetic coefficients of variation in t he P/O ratio. Nevertheless, genetic variation in both pollen and ovule production per flower might persist within autogamous as well as outc rossing populations. In autogamous taxa, genotypes with relatively few pollen grains and ovules per flower (but producing relatively high nu mbers of flowers) and genotypes with comparatively high numbers of gam etes per flower (but producing relatively few flowers) could co-exist if lifetime flower production is selectively neutral. In contrast, in outcrossers, the maintenance of genetic variation in ovule and pollen production per flower might result predominantly from their ability to maintain variation in phenotypic and functional gender. Given genetic variation in primary sexual traits, we predict that the genetic corre lation between investment in male and female gametes per flower should qualitatively di?er between selfers and outcrossers. We predict a pos itive genetic correlation between pollen and ovule production per flow er in obligately autogamous taxa, primarily because strong stabilizing selection on the P/O ratio should select against the gender specialis ts that would be necessary to effect a negative genetic correlation be tween mean pollen and ovule production per flower. Moreover, the fact that autogamous individuals are 50% female and 50% male means that gen der-biased phenotypes cannot be functionally gender-biased, preventing selection from favouring phenotypic extremes. In contrast, in outcros sing taxa, in which functionally male-and female-biased genotypes may co-exist, the maintenance of contrasting genders could contribute to t he expression of negative genetic correlations between pollen and ovul e production per flower. We discuss these and a number of corollary pr edictions, and we provide a preliminary empirical test of the first pr ediction.