Stages of Diapause
Embryonic diapause can be divided into three phases. These are entry into diapause, maintenance of diapause, and reactivation (or termination) of diapause.
Entry into Diapause
In tammar wallabies, a species representative of the marsupials, mating occurs the day after birth of offspring. The resulting zygote will grow to be a blastocyst consisting of approximately 100 cells; then development is stopped. This signifies the beginning of embryonic diapause. The body does not automatically suspend growth, however. Embryonic diapause is stimulated by the previous offspring nursing from the mother in what is termed lactational diapause. By the young in the pouch sucking the motherís teat, hormones (described in the hormones and regulation section) are released, halting development of the blastocyst (8).
Maintenance of† Diapause
All together, from suspension of blastocyst development to termination, diapause typically lasts for 11 months of the year in marsupials.
The mechanisms causing this time period vary with species. In marsupials, as mentioned in entry to diapause, it is the suckling stimulus that initially causes diapause. The continued suckling of the neonate is one mechanism that is partially responsible for the maintenance of diapause as well (9). In mustelids, such as the spotted skunk, diapause is maintained by uterine conditions that are insufficient for supporting a pregnancy (5). Once ideal conditions are present, diapause is terminated. In the European badger, there is evidence that poor nutrition can lengthen the span of diapause (3). Although these are all single factors affecting maintenance, research has also lead scientists to believe that the presence of just one inhibitory factor is not enough to maintain diapause (9).
Reactivation (Termination) of† Diapause
Reactivation is the termination of embryonic diapause that allows blastocyst growth and development to resume. In marsupials, this generally occurs around December 22, the summer solstice in the southern hemisphere (read more about why this is ideal in the benefits section). One study experimented with this occurrence, and discovered that a three hour decrease in photoperiod results in birth 29-36 days later, simulating the conditions present around December 22 (10). Based on this information, we can infer that reactivation is photo-periodically induced.
In another interesting study performed by Tyndale-Biscoe et. al in 1974, it was discovered that removing the young, and therefore the stimulus, between February and June causes the blastocyst to resume growth, and the new offspring was born 27 days later. However, if the removal of the young occurred between June and December (during weaning), diapause was not necessarily terminated. This further supports the observation that reactivation is related to season and photoperiod, not just the sucking stimulus. Seasonal diapause is controlled by alterations in the hormone melatonin, as described the hormones and regulation section (13).
The above figure shows the results of a study conducted by Renfree and Tyndale-Biscoe, along with Dr. S. Barker on the months of reactivation of blastocysts (10).
Below is a diagram that brings all of these phases together to show how they fit in between different offspring.