Conservation genetics in hypersaline inland waters: mitochondrial diversity and phylogeography of an endangered Iberian beetle (Coleoptera: Hydraenidae)

Role of climatic variability in shaping intraspecific variation of thermal tolerance in Mediterranean water beetles

The climatic variability hypothesis (CVH) predicts that organisms in more thermally variable environments have wider thermal breadths and higher thermal plasticity than those from more stable environments. However, due to evolutionary trade-offs, taxa with greater absolute thermal limits may have little plasticity of such limits (trade-off hypothesis). The CVH assumes that climatic variability is the ultimate driver of thermal tolerance variation across latitudinal and altitudinal gradients, but average temperature also varies along such gradients. We explored intraspecific variation of thermal tolerance in three typical Mediterranean saline water beetles (families Hydrophilidae and Dytiscidae). For each species, we compared two populations where the species coexist, with similar annual mean temperature but contrasting thermal variability (continental vs. coastal population). We estimated thermal limits of adults from each population, previously acclimated at 17, 20, or 25 °C. We found species-specific patterns but overall, our results agree with the CVH regarding thermal ranges, which were wider in the continental (more variable) population. In the two hydrophilid species, this came at the cost of losing plasticity of the upper thermal limit in this population, supporting the trade-off hypothesis, but not in the dytiscid one. Our results support the role of local adaptation to thermal variability and trade-offs between basal tolerance and physiological plasticity in shaping thermal tolerance in aquatic ectotherms, but also suggest that intraspecific variation of thermal tolerance does not fit a general pattern among aquatic insects. Overlooking such intraspecific variation could lead to inaccurate predictions of the vulnerability of aquatic insects to global warming.

Plasticity of thermal performance curves in a narrow range endemic water beetle

Thermal history can plastically alter the response of ectotherms to temperature, and thermal performance curves (TPCs) are powerful tools for exploring how organismal-level performance varies with temperature. Plasticity in TPCs may be favoured in thermally variable habitats, where it can result in fitness benefits. However, thermal physiology remains insufficiently studied for freshwater insects despite freshwater biodiversity being at great risk under global change. Here, we assess how acclimation at either summer or winter average temperatures changes TPCs for locomotion activity and metabolism in Enochrus jesusarribasi (Hydrophilidae), a water beetle endemic to shallow saline streams in SE Spain. This beetle is a bimodal gas exchanger and so we also assessed how aerial and aquatic gas exchange varied across temperatures for both acclimation treatments. Responses of locomotory TPCs to thermal acclimation were relatively weak, but high temperature acclimated beetles tended to exhibit higher maximum locomotor activity and reduced TPC breadth than those acclimated at lower temperature. High temperature acclimation increased the thermal sensitivity of metabolic rates, contrary to the response generally found in aquatic organisms. Higher metabolic rates upon high temperature acclimation were achieved by increasing aerial, rather than aquatic oxygen uptake. Such plastic respiratory behaviour likely contributed to enhanced locomotor performance at temperatures around the optimum and thermal plasticity could thus be an important component in the response of aquatic insects to climate change. However, high temperature acclimation appeared to be detrimental for locomotion in subsequent exposure at upper sublethal temperatures, suggesting that this narrow range endemic may be vulnerable to future climate warming. This study demonstrates that TPCs are context-specific, differing with performance metric as well as thermal history. Such context dependency must be considered when using TPCs to predict organismal responses to climate change.

Underestimated diversity: Cryptic species and phylogenetic relationships in the subgenus Cobalius (Coleoptera: Hydraenidae) from marine rockpools

Marine rockpools are isolated patches of habitat in the supratidal environment (the so-called splash zone), at the transition between sea and land, found along the rocky shores worldwide and characterized by harsh conditions for life. Nonetheless, few specialized invertebrates successfully colonized this peculiar environment. Among them several members of the water beetles Ochthebius Leach, 1815, subgenus Cobalius Rey (1886), which are found almost exclusively in supratidal and upper-most intertidal marine rockpools from the eastern Atlantic Ocean (Cape Verde, Canary Islands and Morocco, Madeira, Azores) throughout the whole Mediterranean basin. The subgenus Cobalius before 2020 was considered to include ten valid species, based on morphological differences. In late 2020, four additional new species were described. However, recent molecular phylogenetic studies have uncovered further cryptic diversity suggesting the presence of multiple undetected species within this group, highlighting that the species boundaries remain unclear and systematics and taxonomy are in need of revision. In this study we provide a molecular phylogeny based on DNA sequence data from mitochondrial and nuclear genes obtained from ten described species belonging to the subgenus Cobalius, and extensive taxon sampling, in order to better understand the phylogenetic relationships within this genus and to infer the biogeographic processes behind its diversification. We also used a molecular clock to define a time window for diversification of distinct clades within the subgenus, and explore aspects of its evolutionary history. Finally, we used three species delimitation methods (PTP, GMYC and ABGD) to clarify taxonomy and validate species boundaries. Our phylogenetic and biogeographic results identified sixteen independent lineages grouped in four main clades and the possible origin of Cobalius was estimated to be in the Early Miocene (~22 Mya) in W Mediterranean area. Moreover, species delimitation methods suggest there are between 16 and 24 putative species, most of them diverged during the Late Miocene, Pliocene and Pleistocene (6.0-0.11 Mya).

DNA barcoding revels first records of three rare coleopteran genera in Northern lakes of Egypt

One aquatic coleopteran species from family Dytiscidae and two aquatic coleopteran genera from family Hydrophilidae were recorded in the summer period and represent first records in the Egyptian lakes. Beetles were collected from two northern lakes, Lake Idku and Lake Burullus. They were identified by morphological characteristics as well as the mtDNA barcoding method. A molecular phylogenetic approach was used to determine the genetic identity of the collected samples based on the mitochondrial cytochrome oxidase I (COI). Prodaticus servillianus (Dytiscidae) from Egypt showed no significant difference in the COI region and they are highly similar to P. servillianus from Madagascar. The phylogenetic analysis revealed that the other two coleopteran genera belong to family Hydrophilidae. Based on COI only, there is no clear evidence for their genetic identity at the species level. So, we defined them to the closest taxon and denoted them as Cymbiodyta type A and B. The results indicated that resolving the molecular identity of the aquatic beetles from northern lakes of Egypt need more considerations in the field of biological conservation. We concluded that utilization of COI as a barcoding region for identifying some coleopteran species is not sufficient and additional molecular markers are required to uncover the molecular taxonomy at deep levels.

Extreme genetic diversity among springtails (Collembola) in subterranean calcretes of arid Australia

The subterranean islands hypothesis for calcretes of the Yilgarn region in Western Australia applies to many stygobitic (subterranean-aquatic) species that are "trapped" evolutionarily within isolated aquifers due to their aquatic lifestyles. In contrast, little is known about the distribution of terrestrial-subterranean invertebrates associated with the calcretes. We used subterranean Collembola from the Yilgarn calcretes to test the hypothesis that troglobitic species, those inhabiting the subterranean unsaturated (non-aquatic) zone of calcretes, are also restricted in their distribution and represent reciprocally monophyletic and endemic lineages. We used the barcoding fragment of the mtDNA cytochrome c oxidase subunit 1 (COI) gene from 183 individuals to reconstruct the phylogenetic history of the genus Pseudosinella Schäffer (Collembola, Lepidocyrtidae) from 10 calcretes in the Yilgarn. These calcretes represent less than 5% of the total possible calcretes in this region, yet we show that their diversity for subterranean Collembola comprises a minimum of 25 new species. Regionally, multiple levels of diversity exist in Pseudosinella, indicative of a complex evolutionary history for this genus in the Yilgarn. These species have probably been impacted by climatic oscillations, facilitating their dispersal across the landscape. The results represent a small proportion of the undiscovered diversity in Australia's arid zone.

Diversity and distribution of polyphagan water beetles (Coleoptera) in the Lake St Lucia system, South Africa

Water beetles belonging to the suborder Polyphaga vary greatly in larval and adult ecologies, and fulfil important functional roles in shallow-water ecosystems by processing plant material, scavenging and through predation. This study investigates the species richness and composition of aquatic polyphagan assemblages in and around the St Lucia estuarine lake (South Africa), within the iSimangaliso Wetland Park, a UNESCO World Heritage Site. A total of 32 sites were sampled over three consecutive collection trips between 2013 and 2015. The sites encompassed a broad range of aquatic habitats, being representative of the variety of freshwater and estuarine environments present on the St Lucia coastal plain. Thirty-seven polyphagan taxa were recorded during the dedicated surveys of this study, in addition to seven species-level records from historical collections. Most beetles recorded are relatively widespread Afrotropical species and only three are endemic to South Africa. Samples were dominated by members of the Hydrophilidae (27 taxa), one of which was new to science (Hydrobiomorphaperissinottoi Bilton, 2016). Despite the fauna being dominated by relatively widespread taxa, five represent new records for South Africa, highlighting the poor state of knowledge on water beetle distribution patterns in the region. Wetlands within the dense woodland characterising the False Bay region of St Lucia supported a distinct assemblage of polyphagan beetles, whilst sites occurring on the Eastern and Western Shores of Lake St Lucia were very similar in their beetle composition. In line with the Afrotropical region as a whole, the aquatic Polyphaga of St Lucia appear to be less diverse than the Hydradephaga, for which 68 species were recorded during the same period. However, the results of the present study, in conjunction with those for Hydradephaga, show that the iSimangaliso Wetland Park contains a high beetle diversity. The ongoing and future ecological protection of not only the estuarine lake itself, but also surrounding freshwater wetlands, is imperative and should be taken into consideration during future management planning for the park.

Salinized rivers: degraded systems or new habitats for salt-tolerant faunas?

Anthropogenic salinization of rivers is an emerging issue of global concern, with significant adverse effects on biodiversity and ecosystem functioning. Impacts of freshwater salinization on biota are strongly mediated by evolutionary history, as this is a major factor determining species physiological salinity tolerance. Freshwater insects dominate most flowing waters, and the common lotic insect orders Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies) are particularly salt-sensitive. Tolerances of existing taxa, rapid adaption, colonization by novel taxa (from naturally saline environments) and interactions between species will be key drivers of assemblages in saline lotic systems. Here we outline a conceptual framework predicting how communities may change in salinizing rivers. We envision that a relatively small number of taxa will be saline-tolerant and able to colonize salinized rivers (e.g. most naturally saline habitats are lentic; thus potential colonizers would need to adapt to lotic environments), leading to depauperate communities in these environments.

Widespread secondary contact and new glacial refugia in the halophilic rotifer Brachionus plicatilis in the Iberian Peninsula

Small aquatic organisms harbour deep phylogeographic patterns and highly structured populations even at local scales. These patterns indicate restricted gene flow, despite these organisms' high dispersal abilities, and have been explained by a combination of (1) strong founder effects due to rapidly growing populations and very large population sizes, and (2) the development of diapausing egg banks and local adaptation, resulting in low effective gene flow, what is known as the Monopolization hypothesis. In this study, we build up on our understanding of the mitochondrial phylogeography of the halophilic rotifer Brachionus plicatilis in the Iberian Peninsula by both increasing the number of sampled ponds in areas where secondary contact is likely and doubling sample sizes. We analyzed partial mitochondrial sequences of 252 individuals. We found two deep mitochondrial DNA lineages differing in both their genetic diversity and the complexity of their phylogeographic structure. Our analyses suggest that several events of secondary contact between clades occurred after their expansion from glacial refugia. We found a pattern of isolation-by-distance, which we interpret as being the result of historical colonization events. We propose the existence of at least one glacial refugium in the SE of the Iberian Peninsula. Our findings challenge predictions of the Monopolization hypothesis, since coexistence (i.e., secondary contact) of divergent lineages in some ponds in the Iberian Peninsula is common. Our results indicate that phylogeographic structures in small organisms can be very complex and that gene flow between diverse lineages after population establishment can indeed occur.

Population genetics of Mioscirtus wagneri, a grasshopper showing a highly fragmented distribution

The genetic consequences of population fragmentation and isolation are major issues in conservation biology. In this study we analyse the genetic variability and structure of the Iberian populations of Mioscirtus wagneri, a specialized grasshopper exclusively inhabiting highly fragmented hypersaline low grounds. For this purpose we have used seven species-specific microsatellite markers to type 478 individuals from 24 localities and obtain accurate estimates of their genetic variability. Genetic diversity was relatively low and we detected genetic signatures suggesting that certain populations of M. wagneri have probably passed through severe demographic bottlenecks. We have found that the populations of this grasshopper show a strong genetic structure even at small geographical scales, indicating that they mostly behave as isolated populations with low levels of gene flow among them. Thus, several populations can be regarded as independent and genetically differentiated units which require adequate conservation strategies to avoid eventual extinctions that in highly isolated localities are not likely to be compensated for with the arrival of immigrants from neighbouring populations. Overall, our results show that these populations probably represent the 'fragments' of a formerly more widespread population and highlight the importance of protecting Iberian hypersaline environments due to the high number of rare and endangered species they sustain.

Phylogeography and local endemism of the native Mediterranean brine shrimp Artemia salina (Branchiopoda: Anostraca)

There has been a recent appreciation of the ecological impacts of zooplanktonic species invasions. The North American brine shrimp Artemia franciscana is one such alien invader in hyper-saline water ecosystems at a global scale. It has been shown to outcompete native Artemia species, leading to their local extinction. We used partial sequences of the mitochondrial Cytochrome c Oxidase Subunit 1 (COI or cox1) gene to investigate the genetic diversity and phylogeography of A. salina, an extreme halophilic sexual brine shrimp, over its known distribution range (Mediterranean Basin and South Africa) and to assess the extent of local endemism, the degree of population structure and the potential impact of traditional human saltpan management on this species. We also examined the phylogenetic relationships in the genus Artemia using COI sequences. Our results show extensive regional endemism and indicate an early Pleistocene expansion of A. salina in the Mediterranean Basin. Subsequent population isolation in a mosaic of Pleistocene refugia is suggested, with two or three refugia located in the Iberian Peninsula. Two instances of long-distance colonization were also observed. Surprisingly, given its strong phylogeographical structure, A. salina showed a signature of correlation between geographical and genetic distance. Owing to strong 'priority effects', extensive population differentiation is retained, despite dispersal via migrant birds and human management of saltpans. The foreseeable expansion of A. franciscana is likely to be followed by substantial loss of genetic diversity in Mediterranean A. salina. Large genetic divergences between Mediterranean and South African A. salina suggest that the latter deserves species status.