HILEMAN, LENA C.1*, ELENA M. KRAMER1, and DAVID A. BAUM2. 1Department of Organismic and Evolutionary Biology, Harvard University, Cambridge MA 02138 USA; 2Department of Botany, University of Wisconsin, Madison WI 52706 USA. - Floral symmetry genes are implicated in the developmental evolution of stamen number in the Antirrhineae (Veronicaceae).
In the tribe Antirrhineae there has been a single evolutionary
transition from the ancestral condition of four mature stamens per
flower (found in Antirrhinum majus) to the derived condition of
two mature stamens (found in Mohavea). Wild-type A.
majus flowers initiate five stamens, however the adaxial stamen
aborts early in development. Molecular genetic work with A.
majus has shown that the floral symmetry genes CYCLOIDEA
(CYC) and DICHOTOMA (DICH) determine adaxial
flower identity and that CYC is necessary for stamen abortion.
Mohavea is a traditionally recognized genus of two species,
M. confertiflora and M. breviflora, embedded within a
tetraploid clade of North American Antirrhinum. The flowers of
Mohavea develop similarly to those of A. majus except
stamen abortion is seen in the two lateral, in addition to the adaxial
stamen. Using mRNA in situ hybridization, we show that the
M. confertiflora homologs of CYC (McCYC1 and
McCYC2) and DICH (McDICH1) are expressed in both
dorsal and lateral stamen primordia during M. confertiflora
flower development. The evolutionary loss of lateral stamens has
likely been caused by changes in the spatial regulation of these loci.
Expression data suggest the possibility that McDICH2 is under
relaxed purifying selection relative to McDICH1. Patterns of
molecular evolution across tetraploid North American
Antirrhinum, including Mohavea, do not support this
hypothesis and suggest instead that both CYC and both
DICH loci are under purifying selection following their origins
via genome duplication.
Key words: Antirrhineae, CYCLOIDEA, DICHOTOMA, tetraploid evolution