Comparative Developmental Study of the Evolutionary Reduction of Adaxial Microsporangia in Microseris bigelovii (Asteraceae)

Abstract

About 10% of all angiosperm species have only 2 pollen sacs (microsporangia) per anther in place of the normal 4. The reduction seems to have occurred independently in about 50 angiosperm families and is a prime example of the parallel evolution of a potentially diagnostic taxonomic character (homoplasy). In the genus Microseris reduction from 4 to 2 microporangia (MS) is a synapomorphy for three of the diploid annual species. We have compared anther development in the tetrasporangiate (4MS) M. douglasii with that in the disporangiate (2MS) M. bigelovii. In this case, the divergence between the developmental pathways becomes visible late in ontogeny, when the adaxial archesporial cells divide into primary parietal cells and sporogenous cells. In the disporangiate M. bigelovii this differentiation step is lacking so that no adaxial MS are formed. Anther development in a recombinant inbred derivative of a hybrid between M. douglasii and M. bigelovii resulted in the formation of sterile adaxial MS. This suggests a two-step evolution of the character: suppression of the differentiation of fertile cells in otherwise normal adaxial loculi followed by the suppression of the production of sterile precursors. Comparison with other cases of evolutionary reduction of microsporangia suggests that there are (many) different ways to reach the same stable (“canalized”) phenotype. This may be generally true for homoplasy in complex developmental characters.

Key words

Microseris - anther development - organ numbers - diagnostic characters