Ongoing Speciation and Gene Flow between Taxonomically Challenging Trochulus Species Complex (Gastropoda: Hygromiidae)

Abstract

Geographical isolation, selection and genetic drift can cause the geographical diversification of populations and lead to speciation. Land snail species in the genus Trochulus show overlaps in geographical ranges as well as in morphology, but genetic data do not always support the species-level taxonomy based on morphological characters. Such a group offers an excellent opportunity to explore the processes involved. We have addressed the problem by determining the status of the restricted endemic T. graminicola within the larger context of Trochulus taxonomy. We used an integrated approach based on morphological features, ecological preferences and two molecular markers: mitochondrial COI sequences and microsatellites. Comparison of these results demonstrated: (i) conchological distinction of T. striolatus and T. sericeus; (ii) anatomical, ecological and genetic differentiation of T. graminicola and (iii) concordance between morphological characters and mtDNA markers in T. striolatus. Moreover, our data showed an intricate evolutionary history within the genus Trochulus, which can be best explained by: (i) recent or ongoing gene flow between taxa or (ii) their large ancestral polymorphism. Both of these hypotheses suggest that diversification within this group of snails has occurred relatively recently. The mismatches between species defined on morphology and on molecular genetics indicate the complexity of the processes involved in the diversification of this genus.

Speciation in sympatric species of land snails from the genus Trochulus (Gastropoda, Hygromiidae)

“…Our integrative study allowed to recognise it as a separate taxon. Previous phylogenetic analyses based on the COI gene showed that T. coelomphala is most closely related to T. graminicola (Falkner, 1973), an endemic species known only from its type locality in south-western Germany, about 200 km away from the T. coelomphala location (Proćków et al, 2017c). However, using four molecular markers, we found that T. coelomphala sequences are grouped within T. hispidus and are closely related with its samples found in the Bavarian Alps in phylogenetic trees (Figure 8, Figures S3 and S4).…”

“…In particular, the T. hispidus complex exemplifies problems in species delimitation. DNA barcoding in the absence of detailed phylogeographic relationships (Duda et al, 2014) has not been successful (Kruckenhauser et al, 2014;Pfenninger et al, 2005;Proćków et al, 2017c). Here, we included four genetic markers in an effort to obtain relationships within and among species of Trochulus with a greater resolution.…”

“…The morphological differences between conspecific populations can be due to environmental or adaptational modifications, which are not congruent with their phylogenetic history. In consequence, estimating species diversity of Trochulus over their distribution range in many cases has proven difficult (Proćków et al, 2013(Proćków et al, , 2014(Proćków et al, , 2017c. For example, the shell size of T. striolatus is mostly a response to prevailing local environmental and/or climate variables, and the shell features do not justify the recognition of its subspecies (Proćków et al, 2017b).…”

“…Although they were later synonymised to Fruticicola hispida (Germain, 1929), the number of species morphologically resembling T. hispidus is far from being agreed (Anderson, 2005;Duda et al, 2014;Welter-Schultes, 2012). Moreover, recent studies have provided an even more confusing picture, revealing that T. sericeus/hispidus constitutes a complex of morphologically similar but genetically divergent species in the Sarine valley in Swiss Western Prealps (Dépraz et al, 2009) and the snails morphologically resembling T. sericeus sometimes form genetically separate clades (Duda et al, 2014;Proćków et al, 2014Proćków et al, , 2017c or conversely, two morphologically different shells assigned to T. hispidus and T. sericeus do not form phylogenetically distinct clades (Duda et al, 2014;Proćków et al, 2013); hence, these snails are often described as the T. hispidus complex (Kruckenhauser et al, 2014). Furthermore, T. hispidus and T. sericeus appeared to be phenotypically plastic and showed no interbreeding constraints (Proćków et al, 2017a).…”

“…For example, the shell size of T. striolatus is mostly a response to prevailing local environmental and/or climate variables, and the shell features do not justify the recognition of its subspecies (Proćków et al., 2017b). Recent or ongoing gene flow between some Trochulus taxa may also explain an intricate evolutionary history of this genus (Proćków et al., 2017c).…”