Seasonal patterns in serum levels of thyroid hormones and sex steroids in relation to photoperiod-induced changes in spawning time in rainbow trout, Salmo gairdneri.
Serum levels of thyroid hormones (T4 = L-thyroxine and T3 = 3,5,3'-triiodo-L-thyronine) were measured and correlated with previously published levels of 17 beta-estradiol, testosterone (T), and Ca2+ (index of vitellogenin) in four groups of female trout held for 1-3 years at 8.5-9.0 degrees on a ration of 0.5% of body weight day-1 under different constant photoperiod regimes. In group 1, trout under a regimen of 18L:6D became sexually mature (SM) in April/May and then SM again in September/October; in group 2, trout under a regimen of 6L:18D became SM in January/February; in group 3, trout under a regimen of 6L:18D became SM in March/April; in group 4 trout under a regimen of 18L:6D followed by 10L:14D became SM in September/October. In all groups, regardless of the photoperiod-induced changes in spawning time, serum T3 exceeded T4 and both serum T3 and T4 patterns showed a consistent relationship with the sequence of steroid hormone changes and spawning time. Thyroid hormone levels were high during previtellogenesis but fell as sex steroids and serum Ca2+ increased. T3 and T4 were lowest before spawning when sex steroids were at their peak and then increased sharply following spawning as sex steroid levels declined. Peak serum T coincided with ovulation and usually preceded the postreproductive surge in serum T3 and T4. The hypothesis is discussed that T3 enhances early ovarian development, but as energy-demanding vitellogenesis proceeds T3 formation is suppressed, thereby curtailing growth and favoring energy partition to the ova.
Cyr
D G
DG
Department of Zoology, University of Manitoba, Winnipeg, Canada.
Bromage
N R
NR
Duston
J
J
Eales
J G
JG
D000818:Animals; D002118:Calcium / Q000097:blood; D002940:Circadian Rhythm; D003624:Darkness; D004958:Estradiol / Q000097:blood*; D005260:Female; D008027:Light; D012098:Reproduction; D012487:Salmonidae / Q000502:physiology*; D012621:Seasons; D013739:Testosterone / Q000097:blood*; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*; D014337:Trout / Q000502:physiology*
D014284:Triiodothyronine; D013739:Testosterone; D004958:Estradiol; D013974:Thyroxine; D002118:Calcium