Dormancy in Embryos: Insight from Hydrated Encysted Embryos of an Aquatic Invertebrate

Abstract

Numerous aquatic invertebrates remain dormant for decades in a hydrated state as encysted embryos. In search for functional pathways associated with this form of dormancy, we used label-free quantitative proteomics to compare the proteomes of hydrated encysted dormant embryos (resting eggs; RE) with nondormant embryos (amictic eggs; AM) of the rotifer A total of 2631 proteins were identified in rotifer eggs. About 62% proteins showed higher abundance in AM relative to RE (Fold Change>3; = 0.05). Proteins belonging to numerous putative functional pathways showed dramatic changes during dormancy. Most striking were changes in the mitochondria indicating an impeded metabolism. A comparison between the abundance of proteins and their corresponding transcript levels, revealed higher concordance for RE than for AM. Surprisingly, numerous highly abundant dormancy related proteins show corresponding high mRNA levels in metabolically inactive RE. As these mRNAs and proteins degrade at the time of exit from dormancy they may serve as a source of nucleotides and amino acids during the exit from dormancy. Because proteome analyses point to a similarity in functional pathways of hydrated RE and desiccated life forms, REs were dried. Similar hatching and reproductive rates were found for wet and dried REs, suggesting analogous pathways for long-term survival in wet or dry forms. Analysis by KEGG pathways revealed a few general strategies for dormancy, proposing an explanation for the low transcriptional similarity among dormancies across species, despite the resemblance in physiological phenotypes.

Metabolomics reveals novel insight on dormancy of aquatic invertebrate encysted embryos

“…Some candidates for these NADES were at higher abundance in AM in comparison with RE. Trehalose, with an established role in the formation of a glass at ambient temperatures was previously reported to occur at low levels in rotifer RE 31 ; transcriptome and proteome profiling studies revealed the occurrence of enzymes in the trehalose synthesis pathway 17,32,62 . Our current results generally indicate a higher abundance of trehalose in RE, suggesting a functional role for this metabolite in dormant RE, in contrast to a previous publication 31 .…”

“…However, it was not detected in desiccated bdelloid rotifers and tardigrades 16,30 . Only small amounts of trehalose (0.35% of the dry weight) were found in rotifer RE 31 , although two transcripts encoding for trehalase-6-phophate synthase, and four trehalose phosphate synthase enzymes, were detected in rotifer RE 17,32 , suggesting its biosynthesis in rotifers.…”

“…In organisms where dormancy is associated with desiccation, there is a formation of a glassy or vitrified state, which entails an increase in cellular viscosity and leads to a dramatic reduction in biochemical reactions, including metabolism and other functional pathways 13 . A few developmental stages and/or adult invertebrate species survive desiccation for numerous years, including nematodes, bdelloid rotifers and tardigrades 14–16 , rotifer RE 17–20 and insects 21–24 . Prior to desiccation in various organisms, numerous metabolites accumulate that may ameliorate the harmful effects of dehydration, including sugars such as glucose and trehalose, polyols as mannitol, erythritol and glycerol, as well as amino acids 5,25–28 .…”

“…Numerous studies describe transcripts and proteins in association with dormancy but there is little transcriptional similarity among dormancies across species 43 and diverse transcriptional strategies for producing them were reported in insects 44 (and references cited in this publication). The similarity in phenotypes can be partially explained by proteome profiling, which shows a low correlation with transcriptomics 17 . Metabolomics holds promise to offer additional information on dormancy as metabolites are downstream of gene transcripts and have a key impact on protein expression 45,46 .…”

“…Moreover, features with putative functions in signal transduction pathways were also identified. Since rotifer RE are tolerant to desiccation 17–20,37 , we also searched for small molecules, with a possible role in the formation of a glass and protection of membranes and proteins. The current study demonstrates the functional importance of metabolomics in complimenting transcriptomics and proteomics in deciphering the molecular basis underlying dormancy.…”