Gut microbiome diversity has been strongly associated with mood-relating behaviours, including major depressive disorder (MDD). This association stems from the recently characterised bi-directional communication system between the gut and the brain, mediated by neuroimmune, neuroendocrine and sensory neural pathways. While the link between gut microbiome and depression is well supported by research, a major question needing to be addressed is the causality in the connection between the two, which will support the understanding of the role that the gut microbiota play in depression. In this article, we address this question by examining a theoretical 'chronology', reviewing the evidence supporting two possible sequences of events. First, we discuss that alterations in the gut microbiota populations of specific species might contribute to depression, and secondly, that depressive states might induce modification of specific gut microbiota species and eventually contribute to more severe depression. The feasibility of both sequences is supported by pre-clinical trials. For instance, research in rodents has shown an onset of depressive behaviour following faecal transplantations from patients with MDD. On the other hand, mental induction of stress and depressive behaviour in rodents resulted in reduced gut microbiota richness and diversity. Synthesis of these chronology dynamics raises important research directions to further understand the role that gut microbiota play in mood-relating behaviours, which holds substantial potential clinical outcomes for persons who experience MDD or related depressive disorders.

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Ewes of seven breeds (Oxford Down, Border Leicester, Finnish Landrace, Southdown, Welsh Mountain, Tasmanian Merino and Soay) were treated with progestagen-treated sponges to synchronize oestrus for an egg-transfer experiment. Ewes of five of the breeds were treated with superovulating doses of pregnant mare serum gonadotropin (PMSG). Mean body weight ranged from 89 kg for Oxford Downs to 20 kg for Soays. Interval from sponge withdrawal to oestrus differed significantly between breeds, ranging from 1-83 days in Finnish Landrace to 3-00 days for Soays, but was not consistently related to ewe weight. Treatment with PMSG prior to sponge removal reduced the interval to oestrus; fertilization rate at the first synchronized oestrus was low. There were marked differences among breeds in proportion of ewes showing oestrus following PMSG treatment, total follicular response, and proportion of follicles ovulated. Ewes of the Finnish Landrace breed, with the highest natural fertility, were equal or superior to the other breeds in every component of response, and yielded more than double the number of fertilized eggs per ewe of the next best breed. Finnish Landrace females 6 to 9 months old made very satisfactory donors. Natural ovulation rates of adult recipient ewes were as follows: Welsh Mountain (23 ewes)-1-43; Border Leicester (15 ewes)-2-00; Finnish Landrace (26 ewes)-3-31.

In an egg-transfer experiment, seven breeds of sheep differing widely in mean mature ewe weight were used: Oxford Down, 96 kg; Border Leicester, 77; Finnish Landrace, 55; Southdown, 52; Tasmanian Merino, 42; Welsh Mountain, 34; and Soay, 22. Body weight and gestation length of recipient ewes are briefly examined. Of 60 recipient ewes, 95% became pregnant. Embryo survival averaged 69% and appeared to be independent of breed of embryo. Litter size of Welsh Mountain, Border Leicester and Finnish Landrace ewes each given four eggs averaged 2-6, 2-9 and 2-9 respectively so that most of the normally observed breed differences are mainly due to differences in ovulation rate. Observed birth weights were in general agreement with those predicted from litter size and donor and recipient ewe weights. The maternal capacity of Finnish ewes was close to average. Litter size affected survival mainly through its effect on birth weight. For Oxford lambs in litters of 3 to 5 from Finnish recipients, birth weight was 42 % that of normal single-born Oxfords and only 42 % survived. For Soays the corresponding values were 59% for birth weight and 85% for survival. Finnish lambs were exceptionally small at birth but had the highest viability of all breeds. Lamb mortality was higher for males (18%) than females (9%) especially in large litters (26 v. 11%) but about the same in both sexes for Finnish lambs.