Thompson Group - Chromatin and Cell Cycle Dynamics in Urochordates
Fig. 1 Oikopleura dioica
in its filter feeding house.
Our laboratory has played a central role in establishing a new marine model organism, Oikopleura dioica (Fig. 1) an abundant planktonic tunicate, and member of the closest extant group to vertebrates. It has a vital role in marine ecosystems and is noted for its ability to rapidly expand population size in response to algal blooms. The house with in which the animal lives, is repetitively synthesized and discarded (every 4 h), with discarded houses representing a significant contribution to global vertical carbon flux in the oceans. Despite being a complex chordate, it has a very compact, sequenced genome of only 70 Mb, smaller than that of the nematode C. elegans and only 2.5% of the human genome.
At the core of these ecologically important features lies a central deviation from that of most of its chordate relatives: rather than growth being principally fueled by proliferative cell cycles, O. dioica, instead, thrives largely via cell growth driven by endoreduplicative cell cycle variants. This permits rapid, near double exponential growth rates, exceeding those found in most metazoans. This strategy is also employed in its ability to rapidly modulate population size. Through an original coenocystic strategy, in which the ovary persists as one giant multinucleate cell, the cyst phase of oogenesis is prolonged to very late in the life cycle, such that reproductive oocyte output can be rapidly and efficiently modulated over 3 orders of magnitude as a function of the nutrient field experienced during its extraordinarily short chordate life cycle.
We conduct research on the two organ systems central to the adaptation of this life history strategy: the oikoplastic epithelium (Fig. 2), responsible for repetitive production of the house, and the ovary (Fig. 3), occupying up to 70% of the total animal volume at maturity. Our objectives are to explore alterations in cell cycle machinery that drive entry into and maintain these diverse endocycling regimes and to investigate how the histone complement of this animal and their posttranslational modifications (PTMs) have evolved in such a compacted gene regulatory landscape. We also pursue efforts towards understanding the origins and mechanisms involved in the cellular templating, construction, and evolution of the complex extracellular house.
Fig 2. Cellular fields in the oikoplastic epithelium of Oikopleura dioica Left, dorsal view where the epithelium of a day 6 animal has been slit ventrally, isolated from the animal and spread on a slide, anterior at top. Right, lateral view of a whole mount day 4 animal, mouth to the left, gonad at right, tail projecting to bottom right. Nuclei are stained (white) in both images. Specific epithelial cellular fields are indicated by arbitrarily colored nuclei according to the central legend.
Fig. 3. Coenocystic oogenesis in Oikopleura dioica. (I) A day 4 animal (II) Sequential events in oogenesis. The coenocyst is a single giant germline cyst maintained throughout prophase I of meiosis. (III) Confocal images of whole mount female O. dioica (top panels, gonad to right), and cellular details of the coenocyst (bottom panels) at different stages of oogenesis: F-actin, green; DNA, blue. (IV) Schemas of cellular anatomy. The germline coenocyst is surrounded by a single layer of follicle cells, enclosed in a monolayer epithelium. Thousands of germline nuclei are committed to two different fates: half enter meiosis while the other half, termed nurse nuclei (NN), increase ploidy through multiple rounds of endocycling. An internal F-actin network defines pseudo-compartments (pro-oocytes) surrounding each meiotic nucleus (arrowheads in III and MN in IV) open to the general cytoplasm through a persistent 2 µm diameter ring canal. At day 5.5 all meiotic nuclei proceed to a π-conformation of condensed chromatin entering metaphase I. In parallel, a subset of pro-oocytes grows simultaneously by transfer of coenocyst cytoplasm through the ring canal. Only the subset of meiotic nuclei within maturing oocytes continues meiosis. A radical cytoplasmic reorganization ensues, where meiotic nuclei from unselected pro-oocytes gather with nurse nuclei in aggregates. By the end of the life cycle nuclei external to oocytes (NN and MN) undergo apoptosis, leaving a gonad filled with metaphase I arrested oocytes, synchronously released by rupture of the gonad epithelium. Larger view