Genes Suggest Ancestral Colour Polymorphisms Are Shared across Morphologically Cryptic Species in Arctic Bumblebees

email Suzanne orcd idCopyright: © 2015 Williams et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Bombus cullumanus—an extinct European bumblebee species?

International audienceBombus cullumanus s. str. has attracted some of the greatest conservation concerns among bumblebees in Europe because it might now be extinct. However, there has been long-standing disagreement about whether it is conspecific with other eastern pale-banded bumblebees. We investigate these relationships using new data from DNA (COI) barcodes. The results support a Nearctic rufocinctus-group (Bombus rufocinctus) and a Palaearctic cullumanus-group, the latter with just three species: Bombus semenoviellus, Bombus unicus and B. cullumanus s. l. (including several differently coloured taxa). We conclude that, although any persisting B. cullumanus cullumanus s. str. might be a regional conservation priority within Europe, nevertheless, because the species remains common elsewhere within its range in Asia, globally a higher conservation priority should be given to B. unicus, which is genetically more distinct and appears to have a much smaller population in the Russian Far East

The arctic and alpine bumblebees of the subgenus Alpinobombus revised from integrative assessment of species' gene coalescents and morphology (Hymenoptera, Apidae, Bombus)

Williams, Paul H., Berezin, Mikhail V., Cannings, Sydney G., Cederberg, Björn, Ødegaard, Frode, Rasmussen, Claus, Richardson, Leif L., Rykken, Jessica, Sheffield, Cory S., Thanoosing, Chawatat, Byvaltsev, Alexandr M. (2019): The arctic and alpine bumblebees of the subgenus Alpinobombus revised from integrative assessment of species' gene coalescents and morphology (Hymenoptera, Apidae, Bombus). Zootaxa 4625 (1): 1-68, DOI: https://doi.org/10.11646/zootaxa.4625.1.

The arctic and alpine bumblebees of the subgenus Alpinobombus revised from integrative assessment of species’ gene coalescents and morphology (Hymenoptera, Apidae, Bombus)

The bumblebees of the subgenus Alpinobombus of the genus Bombus are unusual among bees for specialising in many of the most northerly vegetated arctic habitats on Earth. Most named taxa in this group (37 available names from a total of 67 names) were described originally from differences in the colour patterns of the hair. Previous revisions have shown unusually little agreement, recognising a range of 6‒9 species, in part because of pronounced intraspecific variation in both skeletal morphology and in the colour patterns of the hair. Here we examine variation among 4622 specimens from throughout the group’s global range. Bayesian inference of the gene tree for the fast evolving mitochondrial COI gene combined with Poisson-tree-process analysis of this tree shows support for 10 gene lineages as candidates for being putative species lineages. Integrative assessment shows that the interpretation of these results is not straightforward. Evidence from the fast evolving mitochondrial 16S ribosomal RNA gene supports two of the COI gene alleles (from the samples B. kluanensis s. str. and ‘unnamed2’) as being associated with just one 16S allele. Double COI bands on the PCR gels for these individuals and double peaks on sequence traces (in one case with both COI alleles sequenced from one individual) identifies this as a likely case of COI paralogy that has resulted in mitochondrial heteroplasmy. Evidence from morphology also supports only the remaining nine lineages as separate. Evidence from extracts of cephalic labial gland secretions (CLGS, with components believed to function as sex pheromones) reported by others shows small diagnostic differences between all of the candidate species examined (although B. kluanensis s. l. was not examined) and shows larger differences between all of the species pairs that we find are likely to have co-occurred at least in the past, revealing a likely limitation to the CLGS approach in cases of recent and continuously allopatric species. Consequently we infer nine species in the subgenus Alpinobombus (so that B. kluanensis s. str. and ‘unnamed2’ are interpreted as conspecific, as B. kluanensis s. l.). We provide distribution maps and identification keys for the nine species. The morphology of the male of B. kluanensis is described for the first time, including a unique, unusually dense pad of short hair on the mandible that may have a function involving CLGS in mate-searching behaviour. In seeking to identify the valid names for these species, seven new lectotypes are designated and support is provided for synonymizing 10 names as proposed in a recent summary table of names. The prevailing usage of Bombus balteatus Dahlbom is maintained as valid by proposing Bombus nivalis Dahlbom and Bombus tricolor Dahlbom as nomina oblita and by proposing Bombus balteatus Dahlbom as a nomen protectum. The prevailing usage of Bombus hyperboreus Schönherr is maintained as valid by supporting Apis arctica Quensel as a nomen oblitum and by supporting Bombus hyperboreus Schönherr as a nomen protectum. We then use sequence data from COI and 16S together with nuclear PEPCK and opsin genes to estimate dated phylogenetic relationships among the nine species, allowing for incongruent gene trees with *BEAST. If crown-group divergence within the subgenus Alpinobombus coincided with the global climate cooling and with the growth of the northern ice sheets at the end of the Miocene at ca 7.2 Ma, then divergences between each of the three pairs of sister species are likely to have coincided with fluctuations in vegetated land connections across the Bering Strait after ca 2.5 Ma.  </jats:p

Evolution of Colour Patterns of the Subgenus <i>Alpinobombus</i>.

<p>Reconstruction of ancestral states by parsimony in Mesquite for two colour-pattern characters each with two states (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144544#pone.0144544.g006" target="_blank">Fig 6</a>) among species based on the estimate of phylogeny in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144544#pone.0144544.g005" target="_blank">Fig 5</a>, several of which are polymorphic and show both states for both characters. Above: banding colour pattern, with yellow spots showing yellow-banded (B) populations, black spots showing unbanded (U) populations, mixed yellow/black spots showing polymorphic populations, mixed yellow/grey spots showing uncertain polymorphic/monomorphic populations (*for <i>B</i>. <i>alpinus</i>, males from the Alps often have a yellow-banded pattern although this is rare among females). Below: tail colour pattern, with orange spots showing populations with pale (P: orange or white) hair on the tail, black spots showing populations with dark (D: black) hair on the tail, mixed orange/black spots showing polymorphic populations.</p

PEPCK polymorphisms.^a

<p>^a Numbers in the top row refer to nucleotide positions within a condensed alignment of the sequences with minimal gaps (903 base pairs), letters are FASTA codes for nucleotides but with additional polymorphisms shown explicitly. Dots indicate a nucleotide matching the first (or for position 732, the second) sequence. Species-diagnostic sites (positions 351 360 533 755 769) are shown in bold, and other polymorphic sites are shown without bold. Numbers next to species’ names are the numbers of sequences examined.</p><p>PEPCK polymorphisms.<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144544#t002fn001" target="_blank">^a</a></p

Recognising Species of the Subgenus <i>Alpinobombus</i> with Bayesian GMYC.

<p>The 173 sequences reduced to 46 unique COI-barcode haplotypes on one BEAST ultrametric gene tree (outgroups not shown). The posterior probability distribution (right, colour scale far right) is plotted against a sample tree from BEAST (left) to provide a ‘heat’ map of the probabilities that haplotypes are conspecific by bGMYC. Black spots show the coalescent node for each species from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144544#pone.0144544.g002" target="_blank">Fig 2</a>.</p

Lists and numbers of Species of the Subgenus <i>Alpinobombus</i> from previous taxonomic revisions.^a

<p>^aSpecies names and spellings are those used in the original publications. The revision by Løken did not cover the New World fauna and so did not treat <i>B</i>. <i>neoboreus / B</i>. <i>strenuus</i>.</p><p>^bCounts of split and lumped species compared to this study exclude the ‘unnamed’ taxon that we treat (conservatively) as unsampled previously.</p><p>Authors of the taxon names: <i>alpinus</i> (Linnaeus), <i>arcticus</i> Kirby, <i>balteatus</i> Dahlbom, <i>hyperboreus</i> Schönherr, <i>kincaidii</i> Cockerell, <i>kirbiellus</i> Dahlbom, <i>natvigi</i> Richards, <i>neoboreus</i> Sladen, <i>polaris</i> Curtis, <i>pyrrhopygus</i> Friese, <i>strenuus</i> Cresson, <i>tristis</i> Friese.</p><p>Lists and numbers of Species of the Subgenus <i>Alpinobombus</i> from previous taxonomic revisions.<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144544#t001fn001" target="_blank">^a</a></p

Estimate of phylogeny for species of the subgenus <i>Alpinobombus</i>.

<p>Estimated using a linked-tree BEAST analysis of COI-barcode and PEPCK exon and intron sequences for each species. Values below the nodes are Bayesian posterior probabilities showing branch support. Numbers above the nodes show the estimated dates of divergence events in Ma (millions of years before the present) calibrated with a molecular estimate for the date of divergence between the subgenus <i>Alpinobombus</i> and the subgenus <i>Bombus s</i>. <i>str</i>. from Hines (2008) and grey bars show the 95% confidence limits on the estimated dates of divergence.</p

Recognising Species of the Subgenus <i>Alpinobombus</i> with Bayesian PTP.

<p>The 46 unique COI-barcode haplotypes as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144544#pone.0144544.g002" target="_blank">Fig 2</a> on a MrBayes metric gene tree with the Bayesian PTP solution with highest support, showing nine prospective species (black spots show the coalescent node for each species; outgroups not shown). The tree is the maximum-clade-credibility tree after 1% burn-in from 100 million generations of the MCMC algorithm in MrBayes. Values above the nodes are PTP Bayesian support values that all daughter haplotypes belong to a single species population; values below the nodes are Bayesian posterior probabilities showing branch support. Haplotype selection and labels as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144544#pone.0144544.g002" target="_blank">Fig 2</a>. The scale bar for branch lengths shows 0.02 substitutions per base position.</p