Evolution of salinity tolerance in the diving beetle tribe Hygrotini (Coleoptera, Dytiscidae)

Abstract

Some species of the diving beetle tribe Hygrotini (subfamily Hydroporinae) are among the few insects able to tolerate saline concentrations more than twice that of seawater. However, the phylogenetic relationships of the species of Hygrotini, and the origin and evolution of tolerance to salinity in this lineage, are unknown. In this work, we aim to reconstruct how many times salinity tolerance did evolve in Hygrotini, whether this evolution was gradual or if tolerance to hypersalinity could evolve directly from strictly freshwater (FW) species, and to estimate the probabilities of transition between habitats. We build a phylogeny with ca. 45% of the 137 species of Hygrotini, including all major lineages and almost all of the known halophile or tolerant species. We used sequence data of four mitochondrial (COI-5′, COI-3′, 16S + tRNA and NADH1) and three nuclear (28S, 18S and H3) gene fragments, plus ecological data to reconstruct the history of the salinity tolerance using Bayesian inference. Our results demonstrate multiple origins of the tolerance to salinity, although most saline and hypersaline species were concentrated in two lineages. The evolution of salinity was gradual, with no direct transitions from FW to hypersaline habitats, but with some reversals from tolerant to FW species. The oldest transition to saline tolerance, at the base of the clade with the highest number of saline species, was dated in the late Eocene-early Oligocene, a period with decreasing temperature and precipitation. This temporal coincidence suggests a link between increased aridity and the development of tolerance to saline waters, in agreement with recent research in other groups of aquatic Coleoptera.

Irreversible habitat specialization does not constrain diversification in hypersaline water beetles

“…The reconstructed habitat transitions supported hypersaline tolerance as an irreversible habitat specialization, with very low estimated rates for the reversals from hypersaline to mesohaline/freshwater habitats (and no reconstructed reversal in the phylogeny). In the context of the limited number of studies to date, our results suggest that unidirectional habitat shifts across the osmotic gradient may be a common feature across different aquatic lineages (Arribas et al., 2014; Pallarés, Arribas, et al, 2017; Villastrigo et al., 2018). Potential trade‐offs between competitive ability and tolerance to osmotic stress have been previously proposed (Herbst, 2001; Latta, Weider, Colbourne, & Pfrender, 2012), where hypersaline waters are argued to provide refugia from competition and/or predation.…”

“…As quantitative data on environmental salinity were sparse, we reconstructed habitat–occupancy using three qualitative states, following previous studies (e.g. Villastrigo et al., 2018): (i) “freshwater,” found exclusively in freshwater environments; (ii) “mesohaline,” can tolerate some range of salinity (including freshwater) but never (or only very occasionally) found in hypersaline environments; and (iii) “hypersaline,” mostly restricted to hypersaline waters (>40 g/L).…”

“…Ochthebius is an ideal candidate for that because, with more than 500 species, is the most speciose lineage of water beetles able to live across the full gradient of salinity, including inland hypersaline streams, coastal rock‐pools and saltpans (Millán et al., 2014; Villastrigo, Jäch, Cardoso, Valladares, & Ribera, 2019). We hypothesize that tolerance to hypersaline waters has evolved multiple times, as in other aquatic beetles (Arribas et al., 2014; Villastrigo et al., 2018), and test its irreversibility and impact on diversification.…”

“…In the nonmarine aquatic realm, water beetles, together with some Diptera and Crustacea, are the only metazoan lineages with representatives across the full gradient of salinity, including species that can stand salt concentrations several‐fold higher than that of the oceans (Picazo, Moreno, & Millán, 2010; Timms & Hammer, 1988). The study of the evolution of salinity tolerance in different water beetles (family Hydrophilidae: Arribas et al., 2014; Pallarés, Arribas, et al, 2017 and Dytiscidae: Villastrigo, Fery, Manuel, Millán, & Ribera, 2018) suggests that specialization to hypersaline waters may be irreversible and has recurrently appeared during periods of global aridification (particularly within the hydrophilid genus Enochrus ), possibly as an exaptation to aridity (Arribas et al., 2014; Pallarés, Arribas, et al, 2017). However, the limited number of species within these studies did not allow investigation of a possible relationship between habitat specialization across salinity gradients and diversification rates.…”

A subterranean species of Exocelina diving beetle from the Malay Peninsula filling a 4,000 km distribution gap between Melanesia and southern China

“…One paratype male of the new species (voucher number IBE-AN1160) was used for a non-destructive DNA extraction using a commercial kit (Qiagen DNeasy Tissue Kit). We successfully amplified six mitochondrial and nuclear genes in five sequencing reactions, two cytochrome c oxidase subunit I fragments (COI-5' -the "barcode"-and COI-3'), 5' end of rrnL RNA plus leucine tRNA transfer (tRNA-L1) plus 5' end of NADH dehydrogenase subunit I (NAD1), and one internal fragment of both small ribosomal unit (18S RNA) and Histone 3 (H3) (see Villastrigo et al 2018, for details of the primers and sequencing conditions). These are fragments routinely used for Dytiscidae systematics.…”