Island Evolution and Systematic Revision of Comoran Snakes: Why and When Subspecies Still Make Sense

<div><p>Species delimitation and species concepts have been a matter of debate among biodiversity researchers in the last decades, resulting in integrative taxonomy approaches and the use of modern species concepts, such as the phylogenetic, evolutionary or general lineage species concepts. The discussion of subspecies status and concepts has been addressed much less extensively, with some researchers completely refraining from recognizing subspecies. However, allopatric insular populations that are particularly differentiated have traditionally been assigned subspecies status. We studied the molecular phylogeny and morphology of endemic Comoran tree snakes of the genus <em>Lycodryas</em>. Taking an integrative taxonomic approach, we used the concept of independent lines of evidence to discriminate between evidence for specific and subspecific status. Molecular (mtDNA) and morphological data provided sufficient evidence to support four different taxa within Comoran <em>Lycodryas</em>. In a revision of this group, we propose two species, each with two subspecies. We present a discussion of the strong sexual dichromatism unique to Comoran <em>Lycodryas</em> within the genus and related genera that may be explained by sexual selection in combination with the absence of major predators. Then, we discuss the effects of insular evolution and the “island rule” on morphological traits in Comoran <em>Lycodryas</em> and in <em>Liophidium mayottensis</em>, another snake endemic to the Comoros. The absence of larger-bodied snakes may have promoted an increase in body size and the number of dorsal scale rows in these species. Finally, we discuss the subspecies concept, its applications and its significance for integrative taxonomy and for limiting taxonomic inflation. We emphasize that taxon descriptions should be based on an integrative approach using several lines of evidence, preferably in combination with statements on the underlying species concepts or operational criteria, to increase the objectivity and comparability of descriptions.</p> </div

CVA plot of meristic data of Comoran <i>Lycodryas</i> species.

<p>Characters included are: V (number of ventral scales), MD (number of middorsal scale rows), SC (number of subcaudal scale rows), SLAB (number of supralabial scales – mean of left and right side), ILAB (number of infralabial scales – mean of left and right side), SLCE (number of supralabials in contact with the eye – mean of left and right side). Note that BMNH 1946.1.21.47, the type specimen of <i>L. maculatus</i>, represents an outlier of the Anjouan sample due to its uniquely low SLAB and uniquely high SC.</p

Phylogram of Comoran and Malagasy <i>Lycodryas</i>.

<p>The tree is based on a Bayesian analysis of a 3498 bp dataset. MrBayes posterior probabilities (*100, bold) and RAxML bootstrap support values (italic) are given above the nodes, TNT jackknife support values are given below the nodes. P-distances of the COI marker, as calculated in MEGA, are given for the entire clade of Comoran <i>Lycodryas</i> and for the two pairs of sister taxa. Note that the threshold for species delimitation in lamprophiid snakes, including <i>Lycodryas</i>, determined in the barcoding of Malagasy reptiles <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042970#pone.0042970-Nagy3" target="_blank">[104]</a> was 8.3% of COI p-distance.</p

Photographs of Comoran <i>Lycodryas</i> specimens.

<p>A: ZSM 38/2010, male, Anjouan. B: ZSM 40/2010, female, Anjouan. C: ZSM 42/2010, male, Mayotte. D: female specimen observed at Boungoundranavi, Mayotte. E: ZSM 43/2010, male, Mohéli. F: ZSM 1682/2008, female, Mohéli. G: ZSM 41/2010, male, Grand Comoro. H: ZSM 703/2000, female, Grand Comoro. Photographs A, B, C, E, G by O. Hawlitschek, D by G. Viscardi, F by B. Brenzinger, H by F. Glaw.</p

Position of the loreal scale in Comoran <i>Lycodryas</i>.

<p>Drawings represent specimens from Anjouan and Mayotte (A) or from Grand Comoro and Mohéli (B). In (A), the loreal scale (Lor) is in contact with supralabials (SL) 2 and 3, whereas in (B) it is in contact only with supralabial 2.</p

Haplotype networks of all molecular markers analyzed.

<p>Colored circles represent haplotypes; larger circles represent haplotypes that are shared by more than one specimen. MtDNA haplotypes that are situated in a common TCS network are connected by black lines, with black dots representing interlying mutation steps. Higher counts of mutation steps are given in numbers. Separate TCS networks are connected by grey lines according to MEGA distance trees. NDNA “networks” were constructed manually.</p

Morphological data and colors of <i>Lycodryas</i> from the four islands of the Comoro archipelago.

<p>SVL = snout-vent length [mm], MD = number of middorsal scale rows at midbody, V = number of ventral scales, SC = number of subcaudal scales, SLAB = number of supralabial scales, ILAB = infralabial scales. In MD, most specimens have 19, but exceptions of 17 were recorded (2 on Grand Comoro, 1 on Mayotte, 2 on Mohéli). No significant morphometrical or meristic differences were detected between sexes on any island.</p>*<p>: 126 in BMNH 1946.1.21.47, type specimen.</p>**<p>: 8 only in BMNH 1946.1.21.47.</p

Success rates of DNA sequencing according to the taxonomy.

<p>Success rates of DNA sequencing according to the taxonomy.</p

Results of the objective clustering analyses of species.

<p>Clustering was conducted in SpeciesIdentifier with arbitrary thresholds of 5% to 15%. The dataset used here contained 130 sequences belonging to 27 species. 6 species were represented by a single sequence. 124 sequences were correctly identified by the 'best match' criterion. *At these clustering thresholds, the 'best closest match' query criterion yielded 1 misidentification.</p

Reliable DNA Barcoding Performance Proved for Species and Island Populations of Comoran Squamate Reptiles

<div><p>In the past decade, DNA barcoding became increasingly common as a method for species identification in biodiversity inventories and related studies. However, mainly due to technical obstacles, squamate reptiles have been the target of few barcoding studies. In this article, we present the results of a DNA barcoding study of squamates of the Comoros archipelago, a poorly studied group of oceanic islands close to and mostly colonized from Madagascar. The barcoding dataset presented here includes 27 of the 29 currently recognized squamate species of the Comoros, including 17 of the 18 endemic species. Some species considered endemic to the Comoros according to current taxonomy were found to cluster with non-Comoran lineages, probably due to poorly resolved taxonomy. All other species for which more than one barcode was obtained corresponded to distinct clusters useful for species identification by barcoding. In most species, even island populations could be distinguished using barcoding. Two cryptic species were identified using the DNA barcoding approach. The obtained barcoding topology, a Bayesian tree based on COI sequences of 5 genera, was compared with available multigene topologies, and in 3 cases, major incongruences between the two topologies became evident. Three of the multigene studies were initiated after initial screening of a preliminary version of the barcoding dataset presented here. We conclude that in the case of the squamates of the Comoros Islands, DNA barcoding has proven a very useful and efficient way of detecting isolated populations and promising starting points for subsequent research.</p></div