Genetics of sex determination in the gynodioecious species Lobelia siphilitica: evidence from two populations.

In order to determine whether interactions between multiple sex-determining genes might be partly responsible for the wide variation in female frequency among populations of Lobelia siphilitica, we used progeny sex ratios from field-collected plants and from controlled crosses within and between two populations. We demonstrate that multiple cytoplasmic male-sterility types are present in a gynodioecious population where female frequency exceeds 50%. These male-sterility types each have corresponding nuclear alleles that can restore pollen fertility. Restoration of one male-sterility type appears to be controlled by a single, dominant allele, but restoration of a second cytoplasmic type is not easily explained with simple genetic models -- perhaps multiple nuclear loci and/or epistatic interactions are involved. In addition, the crosses show that pollen from some hermaphrodite plants in a population containing no females restores male fertility to plants from a geographically distant gynodioecious population that have male-sterile cytoplasm. Furthermore, some plants in that hermaphrodite population carry a male-sterile cytoplasm. Taken together, these results fit theoretical predictions that female frequency might be highly variable among populations when sex is determined by interactions between several nuclear and cytoplasmic genetic factors, some of which may not be present in all populations. The data also illustrate the need for more theoretical and empirical work investigating the evolutionary impact of nuclear restorer genes with complex action, and explaining the existence of nuclear restorers and cytoplasmic male-sterility genes in a population where females are very rare.