Biology Reference
In-Depth Information
labile embryonic and post-hatchling stages only. The juvenile rivulus are
also amenable to 100% feminization (Orlando et al., 2006). These fi eld and
experimental observations clearly indicate that the hermaphrodite has
retained its ability to function as male or female for the last 200,000 years
(Tatarentov et al., 2009), and has successfully escaped the Muller's ratchet.
Their escape from the Muller's ratchet may be traced to the unprecendented
genetic diversity of 1.4 individuals/clones observed among the rivulus
population (Turner et al., 1992). To trace the source of such genetic diversity,
Nakamura Y et al. (2008) have induced 31 artifi cial inseminations between
two clonal strains characterzed by homozygosity and hererozygosity and
found one heterozygous rivulus out of every 13 individuals.
In this context, a publication by Zarkower (2006) becomes relevant. In
the nematode
Caenorhabditis elegans
, there are two natural sexes namely XX
hermaphrodites and XO males. Hermaphrodites are somatic females but
can reproduce either by self-fertilization or by mating with males. Matings
generate broods of 50% XX and 50% XO progenies. Selfi ng progenies are
mostly XX but one out of 500 is a XO male, which is generated due to
non-disjunction of X chromosomes. Whether a similar non-disjunction
of X chromosomes is induced by temperatures below 19
o
C resulting
in the production of 5% (i.e., one out of 20 embryos) primary males in
K. marmoratus
remains to be tested.
Gamete exchanging hermaphrodites: The existence of gamete
exchanging hermaphrodites has been considered to have been limited
to all species belonging to the two genera
Serranus
and
Hypoplectrus
of
Serranidae (Pandian, 2010). Sadovy de Mitcheson and Liu (2008) have
recently confi rmed its existence in another dozen species belonging to
seven families (including Serranidae) in 12 genera. In them, the ovarian
and testicular tissues are separated (by a thin basement membrane e.g.,
Diplectrum formosum
, Bubley and Pashuk, 2010) and with their respective
ducts. Based on the mode of gamete-exchange, there are three groups
within this hermaphroditism. The territorial permanent monogamous (e.g.,
Serranus tortugarum
) and non-territorial serial monogamous (e.g.,
S. tigrinus
)
hermaphrodites mate only as male or female during any specifi c spawning
but they have the ability to switch sex roles between successive spawnings.
In the third group, an accessory structure composed of epithelial cells that
form highly convoluted villi (Fig. 40) is located posterior to the ovotestis;
hydrated oocytes, ovulated once every two days during the spawning
period of 337 days from March to January, are stored to be spawned when
an opposite partner becomes available. The survey of Bubley and Pashuk
(2010) has found 22% individuals of
Diplectrum
formosum
with stored
oocytes in the accessory structure in coastal waters from North Carolina
to Florida, USA.
