The non-marine ostracod Darwinula stevensoni (Ostracoda, Crustaceae) is one of the last remaining examples of an ancient asexual (Schön et al. 2009) and has probably abandoned sexual reproduction approximately 20 million years ago (Straub 1952). In spite of the limited genetic diversity of D. stevensoni, its cosmopolitan distribution in different aquatic habitats indicate that this species can survive asexually in the long-term, possibly because it developed a general purpose genotype (Van Doninck et al. 2002). To unravel the genetic background of this general purpose genotype, we have studied gene activity of D. stevensoni via the RNA sequencing of transcriptomes. The transcriptome is the collection of all active genes which have been transcribed from DNA to RNA under certain conditions. Statistically comparing the genetic composition of transcriptomes allows us to identify differences in active (or expressed) genes, which inform on host adaptations to specific conditions. We characterised transcriptomes of D. stevensoni from natural populations in Spain, England and Belgium. To summarize results in our multi-gene approach, the “gene ontology” (GO) classification system was used which divides genes across groups (“terms”) in accordance to the biological function, molecular pathway or cellular component they affect. Of the 5013 terms which were assessed regarding biological function, 49 were found to have significantly different gene expression between the three different populations. However, none of these terms could be directly linked to temperature-related functions, suggesting that differences in gene expression between natural D. stevensoni populations are more complex and do not only reflect adaptation to varying temperature ranges. Living D. stevensoni individuals from the Belgian population were also exposed to UV-radiation in the lab. In this experiment, we expected to find an increased activity of genes being related to the repair and preservation of DNA countering the effects of UV-radiation. Of the same 5013 analysed gene terms, only 9 showed a significant change in gene expression in response to UV-radiation. None of these gene term had known repair functions, potentially highlighting again more complex gene expression patterns in D. stevensoni than initially expected. More analyses are currently ongoing to unravel the functionality of the activated genes in more detail. We are also planning an experimental set up to directly study gene expression differences of D. stevensoni under different temperatures.
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RBINS Staff Publications 2024
Leatherbacks constitute a bizarre clade of marine turtles today represented by a single species, Dermochelys coriacea. A series of peculiar physiological adaptations and behaviors make this species particularly reminiscent to some marine mammals. These include particularly advanced skeleton adaptations for swimming, the largest body size among living reptiles, highly elevated growth and metabolic rates, and coldwater tolerance, which enable D. coriacea to lead a truly pelagic, highly migratory, cosmopolitan lifestyle. It is one of the deepest diving animal today, which aids searching for its almost exclusive prey of jellyfishes. Due to their pelagic lifestyle and reduced skeleton, however, the fossil record of leatherbacks is very poor. Here we evaluate the skeletal anatomy of Eosphargis gigas from the Ypresian of Belgium, represented by one of the earliest and most completely preserved fossil dermochelyid. E. gigas already shows several of the anatomical specializations of the extant leatherback but it is primitive in retaining a more ossified shell. The autapomorphic rugose surface decoration of the dermal skull indicates high degree of vascularization, which in turn likely aided regulation of acid–base balance relating to hypercapnia (excess blood carbon dioxide) and/or lactate acidosis based on modern and fossil analogies. Both type of acidosis typically occurs during diving and thus E. gigas likely had deep diving capabilities, which is consistent with its postcranial skeleton. The jaw apparatus also shares many specializations with D. coriacea that may represent adaptation for preying on jellyfish. The emerging hypothesis is that the skeletal and physiological adaptations of leatherbacks are all related to feeding specializations and associated deep diving. Many of these evolved early in the lineage under greenhouse climatic conditions with the likely associated deeper placement of the gelatinous plankton zone compared to icehouse conditions. This study was supported by the SYNTHESYS program (grants AT-TAF 1441, FR-TAF 4290, GB-TAF 1882, BE-TAF 5292); Train2Move-Marie Curie Fellowship awarded to M.R.
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RBINS Staff Publications 2019
