FISHBEIN, MARK*, LARRY HUFFORD, AND DOUGLAS E. SOLTIS. School of Biological Sciences, Washington State University, Pullman, WA 99164-4236. - Reversals to hypogyny in Saxifragales: comparative ontogenies and evolutionary correlates.
Saxifragales is a small, but morphologically diverse, clade containing
Saxifragaceae, Crassulaceae, Hamamelidaceae, Haloragaceae,
Paeoniaceae, and related taxa. Of many diverse vegetative and
reproductive characteristics, the range of ovary positions is notable.
This variation, which is manifest at levels ranging from closely
related species to families, appears to violate hypotheses of Stebbins
and Grant that ovary position is an evolutionarily conservative
feature. Apparent reversals from inferior ovaries (epigyny) to
superior ovaries (hypogyny) also contradict widely-held beliefs in an
evolutionary trend towards epigyny; the derivation of hypogyny from
epigyny is considered to be exceedingly rare. We use ontogenetic
criteria established by Kaplan to assess the homology of ovary
position across Saxifragales. SEM and histological study of early
floral buds were used to characterize the conformation of the floral
apical meristem throughout organogenesis. We find that nearly all
clades of Saxifragales are characterized by epigynous development,
regardless of apparent ovary position in mature flowers, affirming the
conservative nature of the developmental basis underlying ovary
position. Apparent variation in ovary position in the clade is due
largely to allometric shifts in the relative rate of growth in the
upper and lower portions of the ovary late in development, rather than
shifts in the conformation of the meristem during the earliest stages
of floral development. However, at least one reversal to hypogynous
development is documented unequivocally; this represents the first
well-documented case of a reversal to hypogyny. We explore
evolutionary changes in other floral characters that are correlated
with changes in ovary position. Reversal to hypogyny in Saxifragales
may be associated with the derivation of apocarpy in the clade,
although the limited number of reversals precludes quantitative
assessment of the association.
Key words: correlated characters, evolutionary reversal, homology, ontogeny, ovary position, Saxifragales