Differential response to heat stress among evolutionary lineages of an aquatic invertebrate species complex

Abstract

Under global warming scenarios, rising temperatures can constitute heat stress to which species may respond differentially. Within a described species, knowledge on cryptic diversity is of further relevance, as different lineages/cryptic species may respond differentially to environmental change. The Brachionus calyciflorus species complex (Rotifera), which was recently described using integrative taxonomy, is an essential component of aquatic ecosystems. Here, we tested the hypothesis that these (formerly cryptic) species differ in their heat tolerance. We assigned 47 clones with nuclear ITS1 (nuITS1) and mitochondrial COI (mtCOI) markers to evolutionary lineages, now named B. calyciflorus sensu stricto (s.s.) and B. fernandoi We selected 15 representative clones and assessed their heat tolerance as a bi-dimensional phenotypic trait affected by both the intensity and duration of heat stress. We found two distinct groups, with B. calyciflorus s.s. clones having higher heat tolerance than the novel species B. fernandoi This apparent temperature specialization among former cryptic species underscores the necessity of a sound species delimitation and assignment, when organismal responses to environmental changes are investigated.

Keywords: Brachionus calyciflorus; critical thermal maximum; cryptic species; ecological speciation; heat tolerance; rotifers.

Figure 1.

(a) Maps of Europe and the USA indicating the origin of clones, merged by country. (b) Bayesian phylogenetic reconstruction of 59 nuITS1 sequences. Midpoint root was used. (c) Bayesian phylogenetic reconstruction of 13 mtCOI haplotypes. Brachionus plicatilis (AY539440) was used as an outgroup. Hap_P01 is present across continents. Numbers in nodes represent the posterior probability. Pies next to alleles and haplotypes represent the place of origin.

Figure 2.

(a) TDT curves obtained under heat stress for 15 clones of former Brachionus calyciflorus species complex. Parameters CT_max and z-value correspond to the intercept and slope of these curves, respectively. (b) The association between CT_max and z-value for all clones. (Online version in colour.)