Dormancy is a well-studied adaptation to facilitate overwintering. Our goal is the identification and characterization of naturally occurring variation affecting reproductive dormancy in Drosophila. An important feature towards this direction is the proper familiarization with and understanding of the trait. This is particularly important for dormancy in Drosophila, given considerable conflict in the literature on different aspects of the trait.
The current definition of dormancy in Drosophila positions the developmental arrest of oogenesis at the previtellogenic stage (stage 7). This distinguishes dormancy from other stress responses (e.g. starvation response), were oogenesis reaches early vitellogenic stages (stages 8 – 9). These early vitellogenic egg chambers are then degenerated to block oogenesis. In an attempt to resolve this, we studied reproductive dormancy in D. melanogaster and D. simulans. Our analyses showed unambiguous evidence for oogenesis block up to early vitellogenic egg chamber stages, regardless of the production of eggs, under dormancy-inducing conditions. Additionally, we observed strong degeneration of early vitellogenic egg chambers. Furthermore, we identified strong interaction between dormancy-inducing conditions and starvation, thus the two stressful conditions should induce the same oogenesis response. Given all these, we propose to redefine dormancy as the block of oogenesis up to early vitellogenic egg chambers (up to stage 9 of oogenesis).
This new definition of dormancy indicates that this cold-temperature response may not differ from other responses. Given evidence from other studies too, it seems that different stress responses share overlapping mechanisms (e.g. the block of oogenesis described above). This corroborates the theory of general stress response against diverse stressful conditions in Drosophila. Finally, we observed that both species express dormancy in cosmopolitan and African/ancestral populations. As a consequence, dormancy may have a more ancestral ancestry and may not have been the presumed determinative adaptation that led to the expansion of the Drosophila distribution.
Having understood dormancy in Drosophila in the physiological and evolutionary level allowed us to proceed to the genetic analysis of the trait with more confidence. We previously found significant differences within and between the two Drosophila species in their sensitivity to cold temperature stress. However, at very extreme temperature stress (8°C) dormancy response was particularly strong and the differences seen at less extreme temperatures were reduced. Thus, we restricted our genetic analysis between 10°C and 12°C dormancy-inducing conditions. At the moment, we screen a single D. simulans population (comprised of more than 1000 isofemale lines) for dormancy at both temperatures. Such a genetic design allows us to achieve homogeneous genetic background and satisfying discrimination between opposite phenotypes. When the screening comes to an end, we will apply whole-genome pool-sequencing to these extreme phenotypes. By comparing their genotypes, we expect to identify the genetic elements that affect dormancy incidence in Drosophila.