Biology Reference
In-Depth Information
highlighted in Table 19. From a long term well designed experiment, Bernet
et al. have undertaken at two sites, one at the natural waters of Lake Thun,
Switzerland and the second one at constant temperature of 8-9°C at a spring
water area. They have traced the ovarian differentiation as interrupted by a
shorter or longer period of resting phase, while the testicular differentiation
once commenced proceeds fast and continuous without any interrupting
resting phase. However, the thermal requirement of about 8,200°D is almost
equal for maturation of male and female, irrespective whether the fi sh have
been reared at constant (8-9°C) temperature, as in spring waters, or at habitat
thermal regime of 4-22°C as in the Lake Thun. Nevertheless, the slow rate
of differentiation in spring waters lasts till maturation; consequently, the
difference in the events of age of sexual differentiation becomes wider and
wider, as they grow. Hence the difference in age at sexual maturity between
the Lake Thun and spring water groups is 303 days or 45% variation. In
terms of size at sexual maturity, this variation amounts to 23% for males
but only 8% for females. Therefore, temperature affects the age at sexual
maturity more effectively than size at sexual maturity; size at male maturity
is more affected than that of the female.
Table 19.
Chronological sequence of events leading to ovarian and testicular differentiation
and sexual maturation in
Corgonus lavaretus
(compiled from Bernet et al., 2009)
Event
At Lake Thun 4 to 22ºC
At Spring water 8-9ºC
Age
(dph)
Size
(cm)
Degree
-days (ºD)
Age
(dph)
Size
(cm)
Degree
-days (ºD)
Undifferentiated gonad
and arrival of PGCs
65
2.0
491
65
2.0
491
Ovarian differentiation
Oocyte development
147
3.5
1820
202
5.5
1734
Perinuclear oocyte
169
7.0
2298
232
6.5
1989
Balbaini body stage
200
7.5
2878
295
7.5
2878
Resting phase
313
640
Vitelligenic oocyte
513
17.5
8356
975
16.5
8305
Matured
678
20.5
8163
981
19.0
8356
Testicular differentiation
Spermatogonia
200
10.0
2878
265
7.5
2270
Spermatocytes
513
17.0
5969
404
9.0
5542
Resting phase
No
No
Spematozoa
513
17.0
5969
404
9.0
5542
Matured with milt
678
19.0
8163
981
15.0
8356
