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DNA Loss and Genome Diminution in a Freshwater Copepod

watercolor and pink M eadax
The freshwater microcrustacean copepod, Mesocyclops edax as it appears in nature, This adult female carries 2 sacs of embryos exterior to her body. At the 5th embryonic cleavage division, embryos excise 80% of their genome from cells destined to form somatic tissues such as intestine and antennae, in a process termed chromatin diminution.

The Wyngaard lab is identifying the molecular mechanisms that underlie the dramatic difference in genome size and architecture between germline and somatic genomes of the zooplankton Mesocyclops edax. This aquatic microcrustacean copepod possesses a diploid adult germline genome size that is five-fold larger than its diploid somatic genome size – a size difference that is in the opposite direction of what occurs in most eukaryotes! A highly programmed excision of DNA, termed “embryonic chromatin diminution,” occurs in the presomatic cell lineage during embryogenesis, dramatically reducing the size of the somatic genome and thought to mark the timing of germline-soma differentiation. We are identifying a diversity of transposable elements (TEs) that comprise the major portion of the excised DNA.

Surprisingly, most of these mobile elements appear to be young and active, pointing to evolutionarily recent proliferations and potentially deleterious activities as they insert into the host genome and disrupt gene functions or expression.  The extraordianry abundance of these elements affords us the opportunity to explore the role of chromatin diminution in genome defense against TE activity. This is a continuation of an undergraduate student-driven research program with the long-term goal of understanding the molecular mechanisms, functional consequences, and evolutionary history of extreme differentiation between germline and somatic genomes.

Pre-diminuted germline and post-diminuted somatic nuclei in Feulgen stained cell squashes of Mesocyclops edax.

A. Adult germ cell nuclei with augmented amounts of DNA
B. Adult somatic nuclei with diminuted amounts of DNA.
C. Chromatin diminution caught in the act duriing a 5th embryonic cleavage division. The 2 postdiminuted daughter cells migrate to opposite poles and enter the somatic lineage. They leave at the metaphase plate their combined, excised DNA, which is resorbed into the nucleoplasm. Image captured by Heidi Dorward, JMU masters student.

Nuclei of 8 – 16 cell embryo prior to chromatin diminution. The smallest, very dense nucleus is the primoridial germ cell. Squash and image prepared by Michelle Clower and Ashton Holub, JMU undergraduate biology majors.