Differentiation in Hermaphrodites - Genetic Sex Differentiation in Fish - page 129

cavity radially or linearly, as in gobiids (Cole and Shaprio, 1992). A unique
feature of the reproductive system of male gobiids is the presence of paired
secretory Accessory Gonadal Structure (AGS) associated with the testis. At
the time of sex change, it arises from the precursive tissue masses (pAGS)
associated with the ventral portion of the ovarian wall in the region of the
common genital sinus. The ovarian cavity is not persistent in the testes of
sex changed gobiids and parapercids (e.g., Parapercis snyderi , Kobayashi
et al., 1993). Among the protoandrics, a thin layer of connective tissue
separates the peripheral testicular and interior ovarian tissues in many
protandrics like Diplodus sargus (Micale and Perdichizzi, 1994). However,
the connective tissue is lacking in A. frenatus (Brusle-Sigard and Reinboth,
1990); male and female germ cells are separated by their respective somatic
Sertoli and follicle cells only, and thereby pose challenging problems to
endocrinologists. For information on the formation and development of
gonadal ducts in the sex changers, Kobayashi et al. (1993), Lo Nostro and
Guerrero (1996) and Alam and Nakamura (2007) may be consulted.
The ontogenetic pathways of sex change in the diandric protogynous
sparid P. pagrus and a protandrus A. schelegeli are illustrated in Fig. 52. As
can be seen, these two hermaphrodites have the delimited gonad type.
In the presence of the delimited type of ovotestis and the consequent 'no
confl ict of hormones' the initial gametogenesis proceeds independently in
the testicular and ovarian zones prior to sexual maturity. Following sexual
maturity, a functional ovary alone is differentiated in the protogynics like P.
pagrus, during the initial female phase; at the onset on sex change, the ovary
is more or less completely regressed, prior to the testicular differentiation
(Kokokiris et al., 2006). Contrastingly, the ovarian and testicular zones of
the protandric A. schlegeli regress and re-differentiate tuned to the milting
and resting seasons during the initial male phase lasting for 2-3 years (Lee
et al., 2008). Surprisingly a similar pattern of alternate regression and re-
differentiation of the testis and ovary is described for protandric Diplodus
sargus possessing a gonad, in which “a thin layer of connective tissue
alone separates the heterosexual zones (Micale and Perdichizzi, 1994) and
possibly in Sparus aurata (Liarte et al. 2007).” Apparently, the processes of
gonadal de-differentiation and re-differentiation occur only once following
the completion of the initial female phase in the protogynics but more
than once during the initial male phase itself in the protandrics (Fig. 53).
The regressed testicular and ovarian zones are left as a small vestigial
structure. Contrastingly, the gonad of secondary sex is simply derived
from the existing wall of the primary gonad in almost all the diandrics
(Fig. 54) and digynics. From the same gonad, which functioned as ovaries
in protogynics and testes in protoandrics, the secondary testes and ovaries,
respectively are derived. Available fi gures on derivation of testis from
the former wall of the undelimited type of the ovary of many protogynic