Genetic differentiation of Elysia timida (Risso, 1818) populations in the Southwest Mediterranean and Mar Menor coastal lagoon

Genetic variation at 10 enzyme loci was analysed in Elysia timida sacoglossan mollusc samples, originating from both coastal lagoon and marine sites. The observed heterozygosity ranged from 0.390 (Los Urrutias) to 0.277 (Tabarca). Marine and coastal lagoon populations were characterised by exclusive alleles.     

Mollusc/algal chloroplast symbiosis: how can isolated chloroplasts continue to function for months in the cytosol of a sea slug in the absence of an algal nucleus?

A marine sea slug, Elysia chlorotica, has acquired the ability to carry out photosynthesis as a result of forming an intracellular symbiotic association with chloroplasts of the chromophytic alga, Vaucheria litorea. The symbiont chloroplasts (kleptoplasts) are functional, i.e. they evolve oxygen and fix CO₂ and actively transcribe and translate proteins for several months in the sea slug cytosol. Considering the dependency of plastid function on nuclear genes, the level of kleptoplast activity observed in the animal cell is quite remarkable. Possible factors contributing to this long-lasting functional association that are considered here include: the presence of an algal nuclear genome in the sea slug, autonomous chloroplasts, unusual chloroplast/protein stability, re-directing of animal proteins to the kleptoplast, and lateral gene transfer. Based on our current understanding, the acquisition and incorporation of intact algal plastids by E. chlorotica is aided by the robustness of the plastids and the long-term functional activity of the kleptoplasts appears to be supported by both plastid and protein stability and contributions from the sea slug.     

Molecular Characterization of the Calvin Cycle Enzyme Phosphoribulokinase in the Stramenopile Alga Vaucheria litorea and the Plastid Hosting Mollusc Elysia chlorotica

Phosphoribulokinase （PRK）, a nuclear-encoded plastid-localized enzyme unique to the photosynthetic carbon reduction （Calvin） cycle, was cloned and characterized from the stramenopile alga Vaucheria litorea. This alga is the source of plastids for the mollusc （sea slug） Elysia chlorotica which enable the animal to survive for months solely by photoautotrophic CO2 fixation. The 1633-bp V. litorea prk gene was cloned and the coding region, found to be interrupted by four introns, encodes a 405-amino acid protein. This protein contains the typical bipartite target sequence expected of nuclearencoded proteins that are directed to complex （i.e. four membrane-bound） algal plastids. De novo synthesis of PRK and enzyme activity were detected in E. chlorotica in spite of having been starved of V. litorea for several months. Unlike the algal enzyme, PRK in the sea slug did not exhibit redox regulation. Two copies of partial PRK-encoding genes were isolated from both sea slug and aposymbiotic sea slug egg DNA using PCR. Each copy contains the nucleotide region spanning exon 1 and part of exon 2 of V litorea prk, including the bipartite targeting peptide. However, the larger prk fragment also includes intron 1. The exon and intron sequences of prk in E. chlorotica and V/itorea are nearly identical. These data suggest that PRK is differentially regulated in V. litorea and E. chlorotica and at least a portion of the V. litorea nuclear PRK gene is present in sea slugs that have been starved for several months.     

海南省首次发现光合软体动物绒毛海天牛北大核心CSCD

本文首次报道了光合软体动物Elysia tomentosa K.R.Jensen,1997在我国海南省的分布情况。2021年9月,本课题组在海南省文昌市云龙湾采集到该物种,通过线粒体基因组核糖体16S rRNA以及组蛋白H3基因(H3)位点部分DNA序列比对,确定该物种为Elysia tomentosa。参考该物种学名并根据其外部形态特征,建议该物种中文学名为“绒毛海天牛”。通过聚合酶链式反应(PCR)鉴定其生活环境中的藻类,确定了该物种共生叶绿体的来源为总状蕨藻(Caulerpa racemose);利用叶绿素荧光诱导曲线,检测了绒毛海天牛的光合荧光特征。这是本课题组在海南省沿海地区发现分布的又一种光合软体动物,进一步丰富了对海南岛沿岸海天牛分布的认知,也为研究光合作用的进化与内共生机理提供新的研究材料。     

海南省海口市发现白边侧足海天牛

本文介绍了一种海天牛(Elysia leucolegnote)在海南岛的新分布情况。E. leucolegnote属于软体动物门囊舌目(Sacoglossa)海天牛超科(Elysioidea)海天牛属。其以某种丝状绿藻(Boodleopsis sp.)为食,从中摄取叶绿体,并具有在动物细胞中保持叶绿体光合活性的独特能力。这种海天牛类群在进化生物学、光合生物学和生理学研究中受到广泛关注。2020年5月,在海南省海口市的东寨港国家级红树林自然保护区的旗调村发现这种海天牛,通过形态对比和分子标记分析确认为E. leucolegnote,因其在国内尚无报道,建议根据学名和外观特征命名为白边侧足海天牛。通过线粒体COⅠ基因的序列比对,构建了我国已知海天牛属物种的进化树。并且在2020年5月到11月间,在白边侧足海天牛生长地跟踪观察并描述其形态特征变化和行为特征。     

Grazing rates of Elysia tomentosa on native and introduced Caulerpa taxifolia

The marine alga Caulerpa taxifolia Vahl (C. Agardh), recognized globally as one of the most prolific non-native species introductions, has been introduced to several temperate locations from where it has since rapidly expanded. C. taxifolia is protected by a toxin (terpenoid) in its tissues that limits grazing by native herbivores. Sacoglossan molluscs of the genus Elysia are among the few organisms that graze C. taxifolia; however, little is known about their feeding ecology. In the current study, we quantified the grazing rates of Elysia tomentosa on native C. taxifolia (Moreton Bay, Queensland) and introduced C. taxifolia (Botany Bay and Lake Conjola, New South Wales). Grazing rates were similar at Moreton Bay sites and Botany Bay; however, they were significantly lower in Lake Conjola. At the maximum observed grazing rate, slugs ate their body weight in C. taxifolia (dry weight) every 18–24 h. Differences in grazing rates between locations may be explained by differences in C. taxifolia morphology rather than native or introduced origin. Handling editor: J. Padisak     

Molecular confirmation of the occurrence of Elysia cf. tomentosa (Mollusca: Heterobranchia) in the Persian Gulf

The molluscan fauna of the Persian Gulf has recently been relatively well documented, yet there are few records of heterobranch sea slugs (opisthobranchs) from the Arabian parts and no report from the Iranian waters. Here we report for the first time the occurrence of one of these molluscs in the northern Persian Gulf (Bandar Abbas, Iran). Sacoglossan specimens were collected in association with the seaweed, Caulerpa sertularioides. Since morphological attributes were not adequately reliable for species identification, molecular approaches were carried out. Maximum-likelihood and Bayesian Inference analysis of partial DNA sequences of the mitochondrial cytochrome c oxidase subunit I (COI) locus were used for DNA barcoding of large-bodied specimens of Elysia. All Persian Gulf specimens were genetically confirmed as Elysia cf. tomentosa sp. 5, one of at least five morphologically similar but genetically distinct species in the taxonomically challenging and unresolved E. tomentosa complex. This species has previously been recorded only from Australia and Thailand and our finding adds another distant point to the geographic distribution of this species.     

Ultrastructure of sequestered chloroplasts in sacoglossan gastropods with differing abilities for plastid uptake and maintenance

Abstract. Many sacoglossan sea slugs incorporate intact, functional chloroplasts from their algal food sources into specialized cells lining the digestive diverticulum. The chloroplasts in adults of Elysia clarki are photosynthetically functional for many months. Members of this species feed on algae in the Ulvophyceae, including species of Penicillus and Bryopsis. However, other sacoglossans (Elysia patina, Elysia rufescens, and Placida kingstoni) use similar algal food sources as do adults of E. clarki, but are unable to maintain the chloroplasts for more than a week, with individuals of P. kingstoni apparently being unable to maintain chloroplasts for >24 h. We have examined chloroplast sequestering cells of these species looking for morphological differences that may help explain the variation in chloroplast sequestration and maintenance among them. Our results indicate that P. kingstoni does not actively sequester chloroplasts at all, digesting them instead. However, the plastid sequestering mechanisms of individuals of E. patina and E. rufescens are similar to those of E. clarki, and the degradation of chloroplasts by specimens of E. patina is ultrastructurally similar to the same process in E. clarki, although chloroplast degradation occurs much more slowly in individuals of E. clarki. Our results suggest that species-level differences in the digestive capability of the phagosomes involved in the uptake of chloroplasts account for variation in the length of these kleptoplastic associations.     

The role of kleptoplasts in the survival rates of Elysia timida (Risso, 1818): (Sacoglossa: Opisthobranchia) during periods of food shortage

The importance of photosynthetic products derived from kleptoplasts in several sacoglossan species is being investigated in different fields, such as chemistry, biomolecular genetics and ecology. This study analyzes the effect of kleptoplasts on the survival rates of Elysia timida by evaluating the development of chlorophyll concentration, total length and survival rates of starved E. timida specimens kept in the light and in the dark. Although chlorophyll concentration values were similar in both cases, after 28 days specimens kept in the dark showed a greater size decrease and a lower survival rate (up to 30% lower) than those kept in the light. It is evident that kleptoplasts provide the mollusc with extra energy at the primary metabolism level to compensate for a shortage in food.