Large-scale gene expression studies in Drosophilamelanogaster
provide important insights into the global relationships that exist between transcription and development [1
]. Profiling the earliest stages of embryonic development through to late adulthood, for example, reveals that up to 90% of transcripts are regulated at some stage during the life-cycle [1
]. The most highly conserved transcriptional components in the embryo often originate maternally and are utilized in developmental processes before zygotic mechanisms have been induced [3
]. Oogenesis is a dynamic, highly regulated process, that depends on the correct developmental, environmental and nutritional cues for normal progression. Here, we perform an extensive analysis of the expression patterns apparent over a 72 hour period during egg development in Drosophila
. Our approach facilitates investigation of dynamics through key transition points in oogenesis by focusing on transcript profiles obtained before and after entry into vitellogenesis.
Egg development in Drosophila
is characterized by a process of nutrient supply to the oocyte via supporting cells [4
]. The earliest stages of oogenesis occur in the germarium, where new egg chambers are produced from germ-line and somatic stem cells. Once egg chambers have left the germarium and entered the vitellarum, they comprise three cell types: the oocyte, nurse cells and somatic follicle cells. Processes within the vitellarium then concern further maturation of these cells, including mechanisms that require the synchronization of processes in the ovary and fat body for successful egg production.
The sequence of developmental events during oogenesis are believed to follow the same overall pattern in species from the holometabolous group of insects, although specializations in ovarian development are often apparent due to differences in the nutritional or ecological challenges faced by different species [5
]. Such specializations include: a) the degree of ovarian development that occurs during pupal stages, b) ovariole number and growth rate, or c) number of eggs produced. Entry into vitellogenesis and the switch from early egg chamber development to more intensive oocyte growth and maturation is, however, recognized as a common control point for progression past early egg development. Vitellogenesis is characterized by a considerable increase in the synthesis of yolk proteins and other cytoplasmic constituents necessary for embryogenesis. Under adverse environmental conditions, ovarian arrest can be an important feature of egg development if continued reproductive effort has little chance of success.
While the induction and physiological state of insects that utilize a reproductive dormancy period are similar among species, the duration and depth of arrest can vary widely. For example, species that display a strong diapausing response initiate a series of hormonally mediated events to prepare and subsequently reduce rates of metabolism and reproduction [6
]. Once the diapausing phase is entered it is not possible for normal activity to resume until the developmental program is completed, regardless of environmental conditions. In contrast, the response of species with a shallow diapause is to reduce growth and prevent the onset of vitellogenesis almost immediately when environmental cues change. This state, sometimes referred to as quiescence, can be rapidly reversed under favourable conditions with little delay [7
When exposed to over-wintering conditions, reproductive dormancy is induced in D. melanogaster
and oogenesis is arrested with egg chambers in a pre-vitellogenic state [8
]. Females remain in this depressed developmental state for several weeks, after which, regardless of the environmental conditions, egg production resumes. At any stage however, it is possible to restart egg development by returning females to normal environmental conditions. In comparison to other species, these characters indicate a somewhat shallow and conditional physiological response to ovarian arrest. Despite this flexibility, D. melanogaster
diapause is under control of the endrocrine system and circadian response. Seasonal fluctuations affecting the light-entrained pacemaker cells of the brain, triggering hormonal conditions necessary for diapause, with D. melanogaster
displaying circadian induced reproductive arrest under laboratory conditions [8
] and over environmental clines [9
]. This physiological state can be terminated by transferring individuals from a short day to long day photoperiod [8
] or by the application of hormones from the endocrine system [11
The immediate hormonal basis for physiologically induced changes during diapause, in most cases, is the absence of Juvenile Hormone [6
]. Juvenile Hormone regulates a number of early female reproductive events to stimulate pre-vitellogenic growth of egg chambers [14
]. However, while responsible for controlling early reproductive development, in D. melanogaster
the deposition of yolk proteins within oocytes and progression through the oogenic cycle is also coincident with increasing titers of ecydsteroids [12
]. Ultimately there is a complex relationship between Juvenile Hormone, ecydsteroids and yolk protein production stemming from environmental cues that influence progression through the oogenic cycle.
Here, we use developmental arrest in D. melanogaster to generate a transcriptional time-course from pre-vitellogenic diapause, through vitellogenesis and into the last stages of egg development within the abdomen. We expect that genes displaying sex-biased transcriptional abundance are sex-biased in function, and by focusing on gender-biased expression within fly abdomens we investigate reproductive dormancy as well as other molecular changes unique to the diapausing female. Of particular interest are those genes activated immediately when egg development enters vitellogenesis. We further identify and discuss co-expressed genes regulated after diapause in light of the underlying site of expression, functions, maternal contribution and evolutionary conservation characteristic of these groups.