Biology Reference
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realizes the expression of sex determination and differentiation genes and
regulates the synthesis and action of the endocrines. Importantly, estrogens
act as an ovarian inducer and play an important role in protogynous sex
change (Nakamura et al., 2003). Aromatase is the critical enzyme that must
be present for biosynthesis of E
2
from testosterone. In contrast, the lack of
steroid hormones, including androgen, plays an important role in testicular
differentiation. All the Nile tilapias treated with aromatase inhibitor (AI,
e.g., tamoxifan, fadrozole) have hermaphroditic gonad with both ovarian
and testicular tissues; clusters of spermatogonial cells distributed around
the outer periphery of the ovigerous lamellae and clusters of Leydig cells
are observed, indicating that “(i) it is possible to induce functional sex
reversal of the gonads, prior to the commencement of sex differentiation,
i.e. the labile period the completion of sex differentiation and (ii) both germ
cells and germ cells supporting somatic cells have still retained the bisexual
potency until the closure of labile period” (Nakamura et al., 2003).
3.8 Morphotypes and differentiation
Diandric and digynic species generate primary males and primary females,
respectively, perhaps to compensate the progressively decreasing male
ratio in protogynic species and female ratio in protandric species (Pandian,
2010). Sperm limitation is known to occur and reduce fertilization success
among pair-spawning species (Shapiro et al., 1994); it has implications to
the probability of egg-sperm encounters, especially where the egg size is
increased resulting in the reduction of fecundity and duration of fl oatation
due to the possible increase in egg density. A primary male may release
7.5 times more sperm than that of a secondary male (Shapiro and Rasotto,
1993) and motility of the sperm of the primary male is several times faster
than that of secondary males (Fitzpatrick et al., 2007). Consequently, the
fertilization success of pair-spawning diandrics such as the tropical wrasse
Halichoeres bivittatus
is 78%, against 85% in the presence of one or more
streaking primary males (Peterson, 1991). Hence the presence of primary
males during 'spawning rush' may ensure a higher fertilization success.
From detailed histological studies, Liu and Sadovy de Mitcheson (2009)
have described major events of gonadal differentiation in monandric and
diandric groupers. An undifferentiated juvenile with biphasic gonad may
choose alternate pathways of sexual differentiation in a diandric grouper:
differentiate directly into a primary male or a functional female (Fig. 55).
Primary males resemble the females in body coloration and appearance but
the secondary males, have a bright body color during the terminal phase
(TP). Shapiro and Rasotta (1993) indicate the instances of primary males of
T. bifasciatum
changing their (intial phase, IP) color into TP males and the
