Determinants of the annual pattern of reproduction in mature male Merino and Suffolk sheep: modification of responses to photoperiod by an annual cycle in food supply.
Rams of a 'Mediterranean breed' (Merino) and a 'temperate breed' (Suffolk) were compared to determine how much of the differences between their reproductive seasons is owing to variation in their responses to photoperiodic and nutritional cues. In a previous study, both nutritional and photoperiodic inputs were held constant, and it was found that the two breeds show similar endogenous rhythms and, when the animals are challenged with a Mediterranean photoperiodic cycle, these endogenous rhythms are similarly modified. The present study tested whether an annual cycle in the supply of forage might modify the patterns that are generated by the interaction between photoperiod and endogenous rhythms. Both breeds were subjected to a simulated 'Mediterranean' annual cycle in photoperiod (10L:14D to 14D:10L) and provided with either constant food supply or a simulated 'Mediterranean' annual cycle in food supply. In Merino rams, testicular growth responded to photoperiod, but nutrition dominated those responses. In Suffolk rams, changes in testicular size can be completely out of phase with changes in body mass because they are driven primarily by photoperiod, with only subtle responses to changes in diet. The cycle of testicular growth in the Suffolk was driven by changes in the secretion of gonadotrophins (follicle-stimulating hormone concentrations and luteinizing hormone pulse frequency). By contrast, in the Merino, the nutritionally driven seasonal cycle of testicular growth was associated primarily with changes in body mass and this relationship could not always be explained by changes in gonadotrophin secretion. Melatonin secretion was not affected by food supply. Thus, the 'Mediterranean' and 'temperate' genotypes have similar endogenous rhythms that are similarly modified by photoperiod but, with respect to seasonal changes in nutrition, they differ in both the nature of their reproductive response and the physiological mechanisms that mediate those responses.
Martin
Graeme B
GB
School of Animal Biology, The University of Western Australia, Crawley, Australia. gmartin@agric.uwa.edu.au
Hötzel
Maria J
MJ
Blache
Dominique
D
Walkden-Brown
Stephen W
SW
Blackberry
Margaret A
MA
Boukhliq
Rachid C
RC
Fisher
James S
JS
Miller
David W
DW
D000818:Animals; D001835:Body Weight; D004032:Diet; D005640:Follicle Stimulating Hormone / Q000378:metabolism; D005523:Food Supply; D007986:Luteinizing Hormone / Q000378:metabolism; D008297:Male; D008550:Melatonin / Q000097:blood; D010507:Periodicity; D017440:Photoperiod; D012098:Reproduction / Q000502:physiology*; D012611:Scrotum / Q000033:anatomy & histology; D012621:Seasons; D012756:Sheep / Q000502:physiology*; D013045:Species Specificity
D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone; D008550:Melatonin