RNA-seq analysis of pregnancy and milk production in the viviparous cockroach, Diploptera punctata

During viviparous reproduction, a mother retains developing progeny inside of her reproductive tract during gestation, after which she gives live birth.  In select species, a mother will provide nutrition beyond that present within the egg to her developing progeny; this is known as matrotrophic viviparity.  Although this type of reproduction is most often associated with mammals, it is occasionally observed in invertebrates such as Diploptera punctata.  Developing progeny of D. punctata are held in a placenta-like brood sac for the duration of a 63 day pregnancy.  During this period, the brood sac epithelium secretes a nutritive milk-like fluid that is consumed by embryos, supporting a nearly six-fold growth. Very little is known about the underlying molecular mechanisms of this process.  This study utilizes RNA-seq analysis to investigate transcriptional changes associated with D. punctata pregnancy.  Following de novo contig assembly, comparison of three stages of pregnancy (pre-lactation, early lactation, late lactation) and males revealed contigs that are differentially expressed between sexes and throughout the female reproductive cycle.  We noted that 434 (2.6%) contigs decreased by five-fold or greater in lactating females.  A comparable number of contigs (350, 2.1%) are increased by more than five-fold during the lactation stage of pregnancy in D. punctata.  This increased group includes previously identified milk genes, which are expressed 100-1000 fold higher during lactation when compared to pre-lactation females.  Further examination of differentially expressed contigs will likely reveal novel underlying mechanisms of D. punctata milk production.