ones, i.e. dioecy does not promote radiation, at least in these plants
(Heilbuth, 2000). The importance of cross-fertilization is made partic-
ularly clear by the fact that even many hermaphroditic animals
mate. Most parasitic Platyhelminthes, i.e. the Monogenea, Trematoda
and Cestoda, are hermaphroditic. Kearn (1970) described mating in the
skin-parasitic monogenean Entobdella soleae; he found no evidence
for self-fertilization (see also Kearn et al., 1993). In the viviparous
Gyrodactylus alexanderi, the first daughter animal is produced without
cross-fertilization, although cross-fertilization may occur in the pro-
duction of the second embryo (Lester and Adams, 1974). Lo (1999)
concluded that mating is the ‘main focus of monogenean life’, based on an
extensive study of a monogenean infecting small tropical reef fish (details
below). Finally, many, if not most, monogeneans have very complex
copulatory organs, and this alone suggests that mating is of extreme
importance.
Didymozoid trematodes and the Chinese liver fluke, parasitic in the
tissues of vertebrates, usually encapsulate in pairs, apparently to ensure
cross-fertilization. Several authors have observed that echinostome
trematodes self-fertilize and produce viable eggs (e.g. Beaver, 1937).
Nollen (1968, 1978, 1990, 1993, 1994a,b, 1996a,b, 1997a,b, 1999, 2000)
made thorough experimental studies of the mating behaviour of
trematodes, using unlabelled flukes and flukes labelled with^3 H-tyrosine
and transplanted into mice or hamsters. He found that three species of eye
flukes (Philophthalmus) never self-inseminated when occurring in groups
(‘restricted mating’), but that several species of echinostomes, as well as
Zygocotyle lunataand Megalodiscus temperatus, either self- or cross-
inseminated when occurring in groups (‘unrestricted mating’). In one
study, Nollen (1996a), using unlabelled worms and worms labelled
with^3 H-tyrosine transplanted into mice and recovered after 5 days, has
shown that, of nine labelledEchinostoma paraensei,four (44%) had self-
inseminated. Of the 39 unlabelled worms found with the labelled ones,
only eight (21%) had cross-inseminated, and six of the labelled worms
(75%) had also self-inseminated. Trouve et al. (1999b) used double
infections withEchinostoma caproniin which the two individuals did
or did not belong to the same isolate from a certain geographical area, and
triple infections in which two of the three individuals originated from
the same isolate and the third one from a different isolate. They found
no differences between intra- and interisolate selfing rates in the first
experiment, and they found preferential outcrossing between individuals
from the same isolate, apparently in order to avoid hybrid breakdown,
in the second experiment. Saito (1984) noted that mostEchinostoma
hortensehad a cirrus inserted into their own metraterm. However, such
observations are not evidence for successful self-insemination. Rohde
(1973) observed that the eggs of three specimens ofLobatostoma manteri
(Aspidogastrea) in the intestine of naturally infectedTrachinotus blochi,
occurring singly, appeared normal, but egg cells had the haploid number
of chromosomes and did not divide at all or development ceased at a very176 K. Rohde