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
