Biology Reference
In-Depth Information
Hemigrammus caudovittatus
prevents the entry of sperm of the widow tetra
Gymnocorymbus ternetzi
(Fig. 23C). Conversely, the short canal with broad
anterior opening in the eggs of
G. ternetzi
readily permits the sperm entry
of
H. caudovitatus
(David and Pandian, 2006).
Interestingly, the chondrostean gametes differ basically from those of
teleost fi sh in having an acrosome in spermatozoa (Psenicka et al., 2007) and
multiple micropyles in eggs (Cherr and Clark, 1985), for instance, 10 in the
hagfi sh
Eptatretus stouti
(Morisawa and Cherr, 2002). Hence the function of
the acrosome, which undergoes acrosomal reaction and fi lament formation
remains unclear, although the acrosome seems to increase the fertilization
process (Psenicka et al., 2011). The mature egg of the paddlefi sh
Polyodon
spathula
has four to 12 micropyles in the animal polar region (Fig. 25). The
sperm entry site in the egg surface under the micropyle consists of tuft
of microvilli. A minute after fertilization, a ball-like enlarged full-grown
fertilization cone is formed, closing the opening of the micropyle (Fig. 25)
and thereby preventing entry of more sperm (Linhart and Kudo, 1997). It
is not clear whether the multiple micropyles with smooth outer surface of
the chondrostean egg are replaced by a single micropyle in teleostean egg
decorated with furrows and/or tile-like architecture to guide the sperm
to the micropyle. A comparative study of the genes responsible for the
development of a single apical micropyle in the egg and the loss of acrosome
in the sperm may prove to be rewarding.
Fig. 25.
Polyodon spathula
: 1 Left panel: Eleven micropyles in a mature egg (220 x). 2 Right
panel: The micropylar area in the fertilized egg 1 minute post fertilization. Note one of the
micropylar canals is sealed with a full-grown fertilization cone (large arrow) and others (arrow
heads) with material, probably cortical alveoli exudates to various degrees (540 x) (from Linhart
and Kudo. 1997. Surface ultrastructure of paddlefi sh eggs before and after fertilization. J Fish
Biol, 51: 533-582, reproduced by permission of The Fisheries Society of the British Isles/John
Wiley & Sons Ltd)
2.11 Secondary sex characters
In many fi shes, males and females are phenotypically very distinct and
these differences often refl ect divergent selective pressures acting on the
sexes. Phenotypic sexual dimorphism refl ects differing patterns of gene
