Faster evolution of a premating reproductive barrier is not associated with faster speciation rates in New World passerine birds

Why are speciation rates so variable across the tree of life? One hypothesis is that this variation is explained by how rapidly reproductive barriers evolve. We tested this hypothesis by conducting a comparative study of the evolution of bird song, a premating barrier to reproduction. Speciation in birds is typically initiated when geographically isolated (allopatric) populations evolve reproductive barriers. We measured the strength of song as a premating barrier between closely related allopatric populations by conducting 2339 field experiments to measure song discrimination for 175 taxon pairs of allopatric or parapatric New World passerine birds, and estimated recent speciation rates from molecular phylogenies. We found evidence that song discrimination is indeed an important reproductive barrier: taxon pairs with high song discrimination in allopatry did not regularly interbreed in parapatry. However, evolutionary rates of song discrimination were not associated with recent speciation rates. Evolutionary rates of song discrimination were also unrelated to latitude or elevation, but species with innate song (suboscines) evolved song discrimination much faster than species with learned song (oscines). We conclude that song is a key premating reproductive barrier in birds, but faster evolution of this reproductive barrier between populations does not consistently result in faster diversification between species.

Introduced European minnow Phoxinus phoxinus in alpine lakes may increase total mercury concentration in brown trout Salmo trutta

In Norway, the cyprinid European minnow Phoxinus phoxinus has been spread far outside its previous natural distribution area, with lots of establishments in mountain lakes where brown trout Salmo trutta originally was the only fish species. We have analysed δ15N and total mercury (THg) concentration in brown trout from eight lakes, situated between 1031 and 1244 m a.s.l. on the Hardangervidda mountain plateau, southern Norway. One of the lakes is inhabited by brown trout and European minnow, while in the other seven lakes, brown trout is the only fish species. δ15N of brown trout were significantly higher in the population with co-existing European minnow, indicating a higher trophic position of brown trout in this population than in the allopatric populations, probably caused by piscivory, as indicated by frequent occurrence of European minnow in brown trout diet. The mercury concentrations in brown trout from this lake had values up to around 0.4 mg THg per kg wet weight. The concentrations were significantly higher than in the lakes without European minnow, and together with the δ15N values, indicating that translocation and establishment of European minnow may increase the trophic position of brown trout in previously allopatric populations, and thereby also increase the mercury level.

Split it up and see: using proxies to highlight divergent inter-populational performances in aquaculture standardised conditions

Background Considering wild inter-populational phenotypic differentiation can facilitate domestication and subsequent production of new species. However, comparing all populations across a species range to identify those exhibiting suitable key traits for aquaculture (KTA; i.e. important for domestication and subsequent production) expressions is not feasible. Therefore, proxies highlighting inter-populational divergences in KTA are needed. The use of such proxies would allow to identify, prior to bioassays, the wild population pairs which are likely to present differentiations in KTA expressions in aquaculture conditions. Here, we assessed the relevance of three alternative proxies: (i) genetic distance, (ii) habitat divergence, and (iii) geographic/hydrologic distances. We performed this evaluation on seven allopatric populations of Perca fluviatilis for which divergences in KTA had already been shown. Results We showed differences in the correlation degree between the alternative proxy-based and KTA-based distance matrices, with the genetic proxy being correlated to the highest number of KTA. However, no proxy was correlated to all inter-populational divergences in KTA. Conclusion For future domestication trials, we suggest using a multi-proxy assessment along with a prioritisation strategy to identify population pairs which are of interest for further evaluation in bioassays.

Allopatric humpback whales of differing generations share call types between foraging and wintering grounds

Humpback whales (Megaptera novaeangliae) are a cosmopolitan baleen whale species with geographically isolated lineages. Despite last sharing an ancestor ~ 2–3 million years ago, Atlantic and Pacific foraging populations share five call types. Whether these call types are also shared between allopatric breeding and foraging populations is unclear, but would provide further evidence that some call types are ubiquitous and fixed. We investigated whether these five call types were present on a contemporary foraging ground (Newfoundland, 2015–2016) and a historic breeding ground (Hawaii, 1981–1982). Calls were classified using aural/visual (AV) characteristics; 16 relevant acoustic variables were measured and a Principal Component Analysis (PCA) was used to examine within-call and between-population variation. To assess whether between-population variation influenced classification, all 16 variables were included in classification and regression tree (CART) and random forest analyses (RF). All five call types were identified in both populations. Between-population variation in combined acoustic variables (PC1, PC2, PC3) was lower within call types than among call types, and high agreement between AV and quantitative classification (CART: 83% agreement; RF: 77% agreement) suggested that acoustic characteristics were more similar within than among call types. Findings indicate that these five call types are shared across allopatric populations, generations, and behavioural contexts.

Gut microbiota in two recently diverged passerine species: evaluating the effects of species identity, habitat use and geographic distance

Background It has been proposed that divergence in the gut microbiota composition between incipient species could contribute to their reproductive isolation. Nevertheless, empirical evidence for the role of gut microbiota in speciation is scarce. Moreover, it is still largely unknown to what extent closely related species in the early stages of speciation differ in their gut microbiota composition, especially in non-mammalian taxa, and which factors drive the divergence. Here we analysed the gut microbiota in two closely related passerine species, the common nightingale (Luscinia megarhynchos) and the thrush nightingale (Luscinia luscinia). The ranges of these two species overlap in a secondary contact zone, where both species occasionally hybridize and where interspecific competition has resulted in habitat use differentiation. Results We analysed the gut microbiota from the proximal, middle and distal part of the small intestine in both sympatric and allopatric populations of the two nightingale species using sequencing of bacterial 16S rRNA. We found small but significant differences in the microbiota composition among the three gut sections. However, the gut microbiota composition in the two nightingale species did not differ significantly between either sympatric or allopatric populations. Most of the observed variation in the gut microbiota composition was explained by inter-individual differences. Conclusions To our knowledge, this is the first attempt to assess the potential role of the gut microbiota in bird speciation. Our results suggest that neither habitat use, nor geographical distance, nor species identity have strong influence on the nightingale gut microbiota composition. This suggests that changes in the gut microbiota composition are unlikely to contribute to reproductive isolation in these passerine birds.

Generalized olfactory detection of floral volatiles in the highly specialized Greya-Lithophragma nursery pollination system

Volatiles are of key importance for host-plant recognition in insects. In the pollination system of Lithophragma flowers and Greya moths, moths are highly specialized on Lithophragma, in which they oviposit and thereby pollinate the flowers. Floral volatiles in Lithophragma are highly variable between species and populations, and moths prefer to oviposit into Lithophragma flowers from populations of the local host species. Here we used gas chromatography coupled with electroantennographic detection (GC-EAD) to test whether Greya moths detect specific key volatiles or respond broadly to many volatiles of Lithophragma flowers. We also addressed whether olfactory detection in Greya moths varies across populations, consistent with a co-evolutionary scenario. We analyzed flower volatile samples from three different species and five populations of Lithophragma occurring across a 1400 km range in the Western USA, and their sympatric female Greya politella moths. We showed that Greya politella detect a broad range of Lithophragma volatiles, with a total of 23 compounds being EAD active. We chemically identified 15 of these, including the chiral 6, 10, 14-trimethylpentadecan-2-one (hexahydrofarnesyl acetone), which was not previously detected in Lithophragma. All investigated Lithophragma species produced the (6R, 10R)-enantiomer of this compound. We showed that Greya moths detected not only volatiles of their local Lithophragma plants, but also those from allopatric populations/species that they not encounter in local populations. In conclusion, the generalized detection of volatiles and a lack of co-divergence between volatiles and olfactory detection may be of selective advantage for moths in tracking hosts with rapidly evolving, chemically diverse floral volatiles.

Polyzosteria cockroaches in Tasmania (Blattodea: Blattidae: Polyzosteriinae) represent a new, endemic species, with allopatric alpine and coastal sub-populations

We describe the endemic Tasmanian cockroach, Polyzosteria yingina sp. nov. (Henry), 78 years after it was first documented. Evidence from morphology, biogeography and CO1 barcodes is used to distinguish this species from related mainland Australian taxa it has previously been confused with. Polyzosteria yingina sp. nov. has two strongly allopatric populations: a compact alpine population above 1000m and a dispersed east coastal one at sealevel. However, mitochondrial Control Region D-loop molecular analysis suggests a single species identity for these disparate populations. Detailed internal and external morphological descriptions and photographs of living and preserved type material are presented. We also speculate on some hypotheses which could account for the unusual distribution of this charismatic insect.