A Role for Molecular Studies in Unveiling the Pathways for Formation of Rotifer Resting Eggs and Their Survival During Dormancy

Abstract

Rotifers are minute aquatic invertebrates inhabiting mainly continental waters with environmental conditions restricting their long-term survival. Formation of dormant forms, diapausing embryos (resting eggs), is a strategy that facilitates their genetic pool survival beyond unfavorable environmental conditions. Resting eggs are formed during the sexual reproductive cycle. Usually, rotifers display cyclic parthenogenesis, with asexually reproducing females (known as amictic females) forming diploid eggs (called amictic eggs) that are responsible for fast population growth. Specific environmental cues known as mixis induce sexual reproduction. Recent studies implicate crowding as the inducing signal for onset of sexual reproduction in brachionid monogonont rotifers. Sexual reproduction starts with the commencement of meiosis leading to the formation of haploid males and followed by the formation of diploid resting eggs. Rotifer resting eggs differ structurally from asexual or amictic eggs and can remain dormant for decades. Following hatching a new cycle of asexual reproduction ensues, leading to population growth and colonization of the specific environmental niche. In some cases, however, sexual reproduction may occur shortly after hatching. Very few molecular studies were performed on monogonont rotifers so far and most were limited to single gene studies. Large scale expressed sequence tags resources were formed recently for the rotifer Brachionus plicatilis. They are providing important platforms for molecular analyses on the events leading to the formation of males, resting eggs and on dormancy in this species. Genes known to be associated with desiccation tolerance during dormancy in other organisms (e.g., yeast spores, Artemia cysts, and seed plants) were also identified in resting eggs, even though they do not necessarily fully desiccate during the dormant period. Future studies should be directed to investigate the regulation of processes associated with the entrance and exit from dormancy, in this important metazoan model.