Biology Reference
In-Depth Information
Thus, Rohde (
1973
) found that the aspidogastrid Lobatostoma manteri,in
single infections, produced only haploid eggs which did not develop
beyond the early blastula. Kearn (
1970
) found no evidence for self-
fertilization in the monogenean Entobdella soleae, and, although the
first daughter animal of the monogenean Gyrodactylus alexanderi is pro-
duced without cross-fertilization, this may then occur in the production
of the second embryo (Lester and Adams,
1974
). The importance of
cross- and self-fertilization have been particularly well studied in digen-
ean trematodes (Nollen
1993
, further references therein). In the
experiments described by Nollen, one species never self-fertilized
even in single infections, others self-fertilized only when isolated, and
others occasionally self-fertilized even in multiple infections. All species
examined cross-inseminated when kept in groups. Mature worms of
one species when transplanted singly, stopped growing and their repro-
ductive system rapidly degenerated. Even in self-fertilizers, it may well
be that cross-fertilization is necessary in the long run, that is, numbers
and/or viability of offspring may be reduced if cross-fertilization does
not occur at least occasionally. Studies over many generations are
necessary to confirm this. Indirect but very strong evidence for the
importance of cross-fertilization in some hermaphroditic helminths is
the amazing complexity of copulatory organs in many monogeneans. It
would be difficult to understand why such a complexity should evolve
in species without cross-fertilization. Many parasites, of course, are
bisexual (copepods, almost all nematodes, isopods, branchiurans, etc.)
and the necessity of cross-insemination is self-evident for these. In some
species, the factors involved in mate finding have been elucidated. Thus,
Fried, Tancer and Fleming (
1980
) studied pairing of the digenean
Echinostoma trivolvis in vitro and found that free sterols were the main
chemoattractants (Fried and Diaz
1987
).
Rohde (
1976a
,
b
;
1977b
;
1979a
;
1980a
;
1984
;
1989
) gave the following
evidence for the ''mating hypothesis'' of niche restriction for parasites:
(1) narrow host ranges and microhabitats lead to increased intraspecific
contact;
(2) adult stages often have fewer hosts and narrower microhabitats than
sexually immature and larval stages;
(3) microhabitats of sessile and rare species are often narrower than those
of more motile and common species; and
(4) microhabitats of some species were shown to become more restricted
at the time of mating.
