Incongruent molecular and morphological variation in the crab spider Synema globosum (Araneae, Thomisidae) in Europe

Abstract

Establishing species boundaries is one of the challenges taxonomists around the world have been tackling for centuries. The relation between intraspecific and interspecific variability is still under discussion and in many taxa it remains understudied. Here the hypothesis of single versus multiple species of the crab spider Synema globosum (Fabricius) is tested. The wide distribution range as well as its high morphological variability makes this species an interesting candidate for re-evaluation using an integrative approach. This study combines information from barcoding, phylogenetic reconstruction based on mitochondrial CO1 and ITS2 of more than 60 specimens collected over a wide range of European localities, and morphology. The findings show deep clades with up to 6% mean pairwise distance in the CO1 barcode without any biogeographical pattern. The nuclear ITS2 gene did not support the CO1 clades. Morphological assessment of somatic and genital characters in males and females and a morphometric analysis of the male palp uncovered high intraspecific variation that does not match the CO1 or ITS2 phylogenies or biogeography either. Screening for endosymbiotic Wolbachia bacteria was conducted and only a single infected specimen was found. Several scenarios might explain these inconsistent patterns. While the deep divergences in the barcoding marker might suggest cryptic or ongoing speciation or geographical isolation in the past, the lack of congruent variation in the nuclear ITS2 gene or the studied morphological character systems, especially the male palp, indicates that S. globosum might simply be highly polymorphic both in terms of its mtDNA and morphology. Therefore, more data on ecology and behaviour and full genome sequences are necessary to ultimately resolve this taxonomically intriguing case.

Resurrection of Synema globosum canariense Dahl, 1907 (Araneae: Thomisidae) with comments on other synonyms within this genus

“…4). All seven sequences from the Canarian specimens were identical, forming a monophyletic sister clade with individuals from mainland Spain, Portugal, and Italy (‘green clade’ in Urfer et al 2021). Uncorrected p-distances between the Canarian clade and the ‘green clade’ range between 0.9-1.6% while distances to the remaining clades ranges between 2.2-3.2% (‘purple clade’ in Urfer et al 2021) and 6.1-6.9% (‘blue clade’ in Urfer et al 2021).…”

“…A fragment of the 5'-COI was successfully amplified and sequenced (GenBank accession numbers OM807203-OM807209) for seven specimens from the Canary Islands. By conducting a maximum-likelihood analysis including those seven sequences as well as sequences obtained in Urfer et al (2021) and sequences from BOLD and GenBank, four main clades were recovered (Fig. 4).…”

“…Urfer et al (2021) studied molecular and morphological variation in 72 adult specimens of S. globosum collected across the European continent and Turkey. The study did not include material from the distributional ranges of the subspecies, S. g. canariense and S. g. nigriventre or from ranges of the possible subspecies, S. g. daghestanicum and S. g. japonicum .…”

“…Somatic and genital characters in both sexes and morphometrics of the male palp exhibited high intraspecific variation that did not match with the genetic data. According to Urfer et al (2021), one possible explanation is that S. g. globosum could be highly polymorphic both in terms of its mtDNA and morphology. Differences in both morphological features compared to mainland specimens as well as the monophyletic position of the Canarian specimens are sufficient arguments to resurrect the subspecies S. g. canariense .…”

“…Genetic structuring in four main clades and the great divergences found among them suggest the potential for high cryptic diversity within S. globosum . An integrative taxonomic approach is suggested to shed light on the diversity of S. globosum and, potentially, to delimit new subspecies, probably fitting with each clade recovered by Urfer et al (2021). Future genetic studies of Synema should also include material from other subspecies and preferentially also other Synema species, and analyses of the nuclear genome for mtDNA lineages in sympatry.…”