Biology Reference
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in them is controlled by a single gene; their size, age at sexual maturity and
morph are genetically determined.
A more complicated but interesting morphotype is found in the
Nigerian cichlid
P. pulcher
(Martin and Taborksy, 1997). The cichlid is
small, sexually dimorphic with color polymorphism. The sex ratio ranges
from 0.33♂ : 0.67♀ in the upper wet region of the Sombreiro River to
0.5♂ : 0.5♀ in the lower dry region. Females are intensely colored with a
purple belly. In males, there are yellow and red colored morphs. These
colors are genetically determined. Red morphs are more aggressive and
about 50% of them are haremic and hold two or more females in each of
their harems. The other 50% red morphs are pair forming males. Individual
red males can change between haremic and pair-forming status but cannot
change into yellow morphs. The yellow morphs develop into pair-forming
males, or become satellites, when the partner or territory is lost. The
yellow morphs cannot change into red ones, although they can change
between paired and satellite status. Apparently, the body color of these
male morphs are sex-linked. However, it is not known which gene(s) in the
Y chromosome induces the differentiation of the body color and a host of
associated behavioral traits (see Pandian, 2011). From the point of genetic
sex determination and differentiation, it is important to know that all the
observed morphotypes among gonochores are phenotypic variants and fall
within a sex, say male or female, i.e., they are all intrasexual morphotypes
only, as these morphotypes do not cross the borders of a sex. Contrastingly,
such phenotypic variants among hermaphrodites are intersexual, as they
do cross the borders of sex.
2.13 Reproductive senescence
As mentioned elsewhere, sex determination refers to that specifi c event
lasting for seconds and minutes only, when a heterogametic gamete fuses
with that of an opposite sex resulting in the development of an ovary
or testis in a bipotential gonad. But the process of sex differentiation is
continued until death of the vertebrates. This consideration opens a larger
issue of whether the fi shes, being poikilotherms, undergo reproductive
senescence or not. Fishes display extraordinary diverse patterns of ageing.
For example, the Pacific salmon undergoes very rapid senescence at
spawning, whereas rockfi shes, characterized by indeterminate growth,
exhibit very slow or negligible ageing (Patnaik et al., 1994). Many small
fi shes with a short life span such as zebrafi sh, medaka and guppy are
characterized by regular time-dependant ageing similar to other vertebrates
(Gerhard et al., 2002). Fishes do accumulate many age-related biomarkers
like β-galactosidase, oxidized proteins and non-degradable lipofuscin
(e.g., Ding et al., 2010), as in other vertebrates. Genetic up-regulation of
