The Evolutionary Reduction of Microsporangia in Microseris(Asteraceae): Transition Genotypes and Phenotypes

Abstract: The loss of the two inner (adaxial) microsporangia (MS) on the anthers is a shared, derived character for three species of the genus Microseris(Asteraceae). In a hybrid between M. douglasii(4 MS) and M. bigelovii(2 MS), one major gene and four modifier loci are responsible for the difference in MS number. The homozygous recessive (2 MS) genotype of the major gene is necessary but not sufficient for the reduction. In addition, at least five M. bigelovii(2 MS) alleles of the three major modifiers are needed for a stable 2‐MS phenotype in all florets of a plant. One, two or three M. bigeloviialleles of the modifiers cause the random reduction or loss of some of the adaxial MS. When the major gene and two modifiers specify 2 MS and only one modifier is homozygous for the M. douglasii(4 MS) alleles, sister plants can have any phenotype from pure 2 MS to pure 4 MS. Here, we examine the phenotypic expression of these genotypes raised under the normal winter annual conditions and under long‐day conditions. In all cases, the phenotypes vary among sister plants, but the range of variation (most notably under long‐day conditions) depends on the specific modifier gene contributing the M. douglasiialleles. The phenotypic variance in one of the genotypes was decreased by a factor of ten in the depauperate heads produced in the long‐day experiment. This effect is mediated by a dependence of the MS phenotype on the position of the floret relative to the edge of the flowering head (capitulum) and directly by the size of the capitulum. Genotypes specifying phenotypes with more or less precisely two or four MS in all florets show hardly any dependence on environmental or developmental factors. The significance of these observations lies in the non‐linear, “canalized” relationship between phenotypic expression and gene dosage, which shows how a qualitative morphological change dependent on a single major gene mutation can pass through a potentially maladaptive intermediate stage.