Analysis of visual pigment absorbance and luminescence emission spectra in marine ostracodes (Crustacea: Ostracoda)

Selection, drift, and constraint in cypridinid luciferases and the diversification of bioluminescent signals in sea fireflies

Understanding the genetic causes of evolutionary diversification is challenging because differences across species are complex, often involving many genes. However, cases where single or few genetic loci affect a trait that varies dramatically across a radiation of species provide tractable opportunities to understand the genetics of diversification. Here, we begin to explore how diversification of bioluminescent signals across species of cypridinid ostracods ("sea fireflies") was influenced by evolution of a single gene, cypridinid-luciferase. In addition to emission spectra ("colour") of bioluminescence from 21 cypridinid species, we report 13 new c-luciferase genes from de novo transcriptomes, including in vitro assays to confirm function of four of those genes. Our comparative analyses suggest some amino acid sites in c-luciferase evolved under episodic diversifying selection and may be associated with changes in both enzyme kinetics and colour, two enzymatic functions that directly impact the phenotype of bioluminescent signals. The analyses also suggest multiple other amino acid positions in c-luciferase evolved neutrally or under purifying selection, and may have impacted the variation of colour of bioluminescent signals across genera. Previous mutagenesis studies at candidate sites show epistatic interactions, which could constrain the evolution of c-luciferase function. This work provides important steps toward understanding the genetic basis of diversification of behavioural signals across multiple species, suggesting different evolutionary processes act at different times during a radiation of species. These results set the stage for additional mutagenesis studies that could explicitly link selection, drift, and constraint to the evolution of phenotypic diversification.

The influence of gravity and light on locomotion and orientation of Heterocypris incongruens and Notodromas monacha (Crustacea, Ostracoda)

For future manned long-d uration space missions, the supply of essentials, such as food, water, and oxygen with the least possible material resupply from Earth is vital. This need could be satisfied utilizing aquatic bioregenerative life support systems (BLSS), as they facilitate recycling and autochthonous production. However, few organisms can cope with the instable environmental conditions and organic pollution potentially prevailing in such BLSS. Ostracoda, however, occur in eu- and even hypertrophic waters, tolerate organic and chemical waste, varying temperatures, salinity, and pH ranges. Thus, according to their natural role, they can link oxygen liberating, autotrophic algae, and higher trophic levels (e.g., fish) probably also in such harsh BLSS. Yet, little is known about how microgravity (µg) affects Ostracoda. In this regard, we investigated locomotion and orientation, as they are involved in locating mating partners and suitable microhabitats, foraging, and escaping predators. Our study shows that Ostracoda exhibit altered activity patterns and locomotion behavior (looping) in µg. The alterations are differentially marked between the studied species (i.e., 2% looping in Notodromas monacha, ~50% in Heterocypris incongruens) and also the thresholds of gravity perception are distinct, although the reasons for these differences remain speculative. Furthermore, neither species acclimates to µg nor orientates by light in µg. However, Ostracoda are still promising candidates for BLSS due to the low looping rate of N. monacha and our findings that the so far analyzed vital functions and life-history parameters of H. incongruens remained similar as under normal gravity conditions despite of its high looping rate.

Despite disorientation in microgravity, seed shrimp could form part of the food chain in artificial ecosystems designed for space missions. Jessica Fischer and Christian Laforsch of Germany’s University of Bayreuth investigated how simulated microgravity changed the movement of two species of seed shrimp (Ostracoda). Ostracoda can cope in unstable conditions and are being considered as food for fish in aquatic bioregenerative life support systems (BLSS) in manned space missions. Fish fed on Ostracoda that eat algae in these systems could form an important source of proteins for humans. Notodromas monacha and Heterocypris incongruens moved around in loops when exposed to simulated microgravity, but N. monacha did not loop as much. Looping movements suggest that gravity is an important orientational cue. Previous studies showed Ostracoda survived and reproduced successfully for months in space despite their looping behavior. Further studies are needed on N. monacha, since its low looping behavior could mean lower energy consumption making it better qualified for BLSS.

Differential expression of duplicated opsin genes in two eyetypes of ostracod crustaceans

In the first molecular study of ostracod (Crustacea) vision, we present partial cDNA sequences of ostracod visual pigment genes (opsins). We found strong support for differential expression of opsins in ostracod median and compound eyes and suggest that photoreceptor specific expression may be a general phenomenon in organisms with multiple receptors. We infer that eye-specific expression predates the divergence of the two species examined, Skogsbergia lerneri and Vargula hilgendorfii, because eye-specific opsin orthologs are present in both species. We found multiple opsin loci in ostracods, estimating that at least eight are present in Skogsbergia lerneri. All opsins from both ostracod species examined are more closely related to each other than to any other known opsin sequences. Because we find no evidence for gene conversion or alternative splicing, we suggest the occurrence of many recent gene duplications. Why ostracods may have retained multiple recent opsin gene duplicates is unknown, but we discuss several possible hypotheses.