WILLIAMS, JOSEPH H. JR.* and W. E. FRIEDMAN. Department of EPO Biology, University of Colorado, Boulder, CO, 80309. - Double fertilization and diploid endosperm in basal angiosperms?
Although the process of double fertilization is a defining feature of
angiosperms, it has only been carefully documented in relatively few,
mostly derived, angiosperm species. Recent phylogenetic analyses have
placed Nymphaeaceae as either the first or second diverging lineage
from the base of the angiosperm tree. We studied the fertilization
biology of Nuphar polysepalum (Nymphaeaceae). Here we confirm
for the first time, using modern tools, the presence of double
fertilization in an angiosperm basal to the monocot and eudicot
clades. Another important and presumed synapomorphy of angiosperms,
virtually unstudied in early angiosperms, is the highly reduced,
monosporic eight-nucleate female gametophyte (embryo sac). The second
fertilization in such an embryo sac is expected to produce a triploid
primary endosperm nucleus, via the fusion of two female polar nuclei
and one male sperm nucleus. Interestingly, the mature Nuphar
embryo sac consists of only four nuclei: two synergids, an egg, and
one polar, or central cell, nucleus. We confirmed the haploid nature
of the central cell nucleus using DNA quantitation of individual
nuclei. The central cell nucleus has the same DNA content as known
haploid nuclei, such as the egg. We further show that, in
cross-pollinated flowers, the products of double fertilization, the
zygote nucleus and the primary endosperm nucleus, have equal initial
DNA quantities, instead of the 2:3 ratio. Thus, contrary to
expectation, the endosperm in one of the earliest angiosperm lineages
is biparental and diploid, not triploid. The lack of information
regarding basic reproductive biology in basal angiosperms represents a
huge gap in our knowledge that may hold further surprises.
Key words: basal angiosperms, character evolution, double fertilization, embryology, female gametophyte, Nymphaceae