Biology Reference
In-Depth Information
and B. splendens are capable of producing 24-50 eggs/g/day (Fig. 30)
and 12-22 eggs/g/day (Fig. 30), respectively. The life time fecundity of
H. atlanticus is about 0.15% of its terminal body weight, and that for the
experimental period of T. zilli is 0.30% of its fi nal body weight. In contrast,
similar estimates for B. splendens and D. rerio are in the range of 1.5-2.6%
of their respective fi nal body weights. Apparently, larger and long living
fi shes allocate more of their energy to somatic growth, whereas the smaller
ones to reproductive output. For instance, a 3-fold increase in body size
decreases the number of eggs by 20% in apogonids (see Pandian, 2010). From
the point of egg and progeny production, aquaculture of smaller fi shes as
baits for sport fi shing and ornamental fi shes for commercial purpose may
prove several times more profi table.
In the context of maintenance of OSCs, a classical observation by
Kaczmarczyk and Koop (2010) in Drosophila melanogaster has relevance.
In this fruitfl y there are early reproducing (S) and late reproducing (L)
strains. Reproduction commences in the S strain from the fi rst week of
adult life, peaks with the fecundity of 10 eggs/day but completely ceases
by the seventh week of adult life. Contrastingly, reproduction in the L
strain peaks to produce 10-12 eggs/day during the 7th week of adult life
and continues even after the ninth week of adult life. On the whole, the life
time fecundity of the early reproducing S strain may not be even a tenth
of late reproducing L strain. Incidentally, both the strains commence with
2.5 OSCs/ovariole. However, by the ninth week of adult life, the S strain
female has only 0.1 OSC/ovariole against 1.94 OSCs/ovariole in the L
strain female. Kaczmarczyk and Koop (2010) have traced the causes for
the effi cient maintenance of OSCs in late life fertility to a few genes. The
signaling pathway of Dpp , a member of TGF-β morphogens, plays a central
role in maintenance of OSCs. The mutant OSCs, that lack Dpp receptors
or transcriptional effectors, undergo premature differentiation (Xie and
Spralding, 2000). A second TGF-β homologue Gbb also contributes to OSCs
maintenance. On the opposite side, the cell-autonomous bag-of-marbles ( bam )
gene acts on OSCs to promote their differentiation. Dpp signaling acts to
prevent bam expression in the OSCs. The activity of Dpp and Gbb signals is
known to decline with age (Zhao et al., 2008). Apparently, mutation in one
or other of these genes may also cause early fertility and reduce the ability
to maintain the OSCs, as age advances.
2.14 Stem cells and Germ cells
Embryonic Stem Cells (ESCs) are known to be present in developing
embryos and adult tissues of multicellular organisms, except perhaps in
pseudocoelomic and haemocoelomic animals (Murugesan et al., 2010).
Like the zygotes, the ESCs generate a large number of cells by continuous
Search WWH ::




Custom Search