Biology Reference
In-Depth Information
2.10 Acrosomeless sperm and micropylar eggs
Among vertebrates, teleost fi shes are unique for the lack of acrosome in
their spermatozoa (Jamieson, 1991). Despite much modifi cation in shape
and mitochondria to suit oviparity and viviparity, the spermatozoa do not
have acrosome (Fig. 22). This feature of lack of the acrosome is unique and
common to teleost fi shes,
albeit
the rudimentary acrosome is reported in the
sturgeon
A. baeri
(Psenicka et al., 2010). Hence, the gene(s) responsible for
the lack of acrosome is conserved among the fi shes. However, no research
is as yet available on this feature.
Fig. 22.
Longitudinal sections of aquasperm of oviparous
Oncorhynchus tshawytscha
(left) and
interosperm of viviparous
Poecilia latipinna
(right). Note the mitochondria in
O. tshawytscha
sperm and the large modifi ed ones in
P. latipinna
(from Jamieson, BGM. 1991.
Fish Evolution
and Systematic Evidence from Spermatozoa,
Cambridge University Press).
Similarly, a single micropyle is present in the egg of fi shes. The sperm
enters through micropyle of the egg (see Pandian, 2010, Fig. 23). This
has facilitated heterospecifi c fertilization in many fi shes (Pandian and
Kirankumar, 2003) and also led to hybridization and polyploidization
(Pandian and Koteeswaran, 1998), which may modify the process of sex
determination and consequently sex differentiation. Hybridization is known
to occur between more than 300 species (Argue and Dunham, 1999). Hence
a detailed account on structure and function of micropyle is provided.
Incidentally, the presence of micropyle in eggs is unique and common
to teleost fi shes; research on identifi cation and characterization of genes
responsible for the formation of micropyle in fi sh eggs is also desired.
