Jamieson, B.G.M., Hodgson, A.N. and Bernard, R.T.F. 1991c. Phylogenetic trends and variation in the ultrastructure of the spermatozoa of sympatric species of South African patellid limpets (Archaeogastropoda, Mollusca). Invertebrate Reproduction and Development 20, 137-146.
Phylogenetic parsimony analysis of sperm
ultrastructure
allows recognition in South African patellid limpets of four
monophyletic
groups: the argenvillei group (3 species); the aphanes
group (2
species); the concolor group (6 species) and a more
heterogeneous cochlear
group (3 Patella species and dubiously associated with these,
species of
the generaHelcion, Nacella and Cellana). Of 5
pairs of
spermatological sister -species, each species is extensively sympatric
with its
sister-species. Sperm of such sympatric species may be morphologically
almost
identical or may show structural differences which would be expected to
block
hybridization. Where, as in the argenvillei and aphanes
groups,
the sperm within each group are structurally similar, it is likely that
immunological and biochemical rather than structural features of the
gametes
act as species barriers preventing hydridization. Where contrasting
morphological specializations of the sperm of sister-species are
demonstrated,
these are considered adaptations for fertilization in concert with but
probably
secondary to immunological/biochemical factors. They are presumed to
augment
rather than initiate reproductive isolation. Examples of such
structural
differences are the development of an elongate anterior acrosomal
process in H.
pruinosus relative to its sister-species H. pectunculus and,
independently, in P. granularis relative to its plesiomorphic
precursor P.
longicosta. Trends in sperm evolution, from a morphology in the
argenvillei
and aphanes groups which is close to that of 'primitive' sperm, include
elongation of the nucleus and development of a pointed conical acrosome
in the
cochlear group; development of a rounded posterior acrosome love in the
concolor group; and numerous other described changes. Patellid species
which
are at least partly sympatric do not maintain reproductive isolation by
staggering of the spawning time in terms of the time of the year,
though the
degree of interspecific, and intraspecific, synchrony during the day
require
further investigation.