Cryptic Diversity of the European Blind Mole Rat Nannospalax leucodon Species Complex: Implications for Conservation

Abstract

We explored the cryptic speciation of the Nannospalax leucodon species complex, characterised by intense karyotype evolution and reduced phenotypic variability that has produced different lineages, out of which 25 are described as chromosomal forms (CFs), so many cryptic species remain unnoticed. Although some of them should be classified as threatened, they lack the official nomenclature necessary to be involved in conservation strategies. Reproductive isolation between seven CFs has previously been demonstrated. To investigate the amount and dynamics of genetic discrepancy that follows chromosomal changes, infer speciation levels, and obtain phylogenetic patterns, we analysed mitochondrial 16S rRNA and MT-CYTB nucleotide polymorphism among 17 CFs-the highest number studied so far. Phylogenetic trees delineated 11 CFs as separate clades. Evolutionary divergence values overlapped with acknowledged higher taxonomic categories, or sometimes exceeded them. The fact that CFs with higher 2n are evolutionary older corresponds to the fusion hypothesis of Nannospalax karyotype evolution. To participate in conservation strategies, N. leucodon classification should follow the biological species concept, and proposed cryptic species should be formally named, despite a lack of classical morphometric discrepancy. We draw attention towards the syrmiensis and montanosyrmiensis CFs, estimated to be endangered/critically endangered, and emphasise the need for detailed monitoring and population survey for other cryptic species.

Keywords: 16S rRNA gene; MT-CYTB gene; chromosomal speciation; cryptic species; evolutionary rates.

Figure 1

Geographic map with the distributional area of N. leucodon (blue shade): Circles—sampling localities of 40 accomplished sequences (IDs in Table 1); squares—sampling localities of all other imported sequences (IDs in Tables S1 and S2). Letters on the map correspond to chromosomal forms (CFs) listed herein and in Table 1, Table S1, and Table S2. N—total sample size of CFs; 2n—diploid chromosomal numbers; NF—fundamental number of chromosomal arms. (A)—Relief location map of Europe (source: https://commons.wikimedia.org/wiki/File:Europe_relief_laea_location_map.jpg accessed on 29 March 2022). (B)—Position of the study area in the European scale (source: https://d-maps.com/ accessed on 30 March 2022).

Figure 2

ML phylogenetic tree inferred from 16S rRNA nucleotide comparison. Since both methods (ML and BI) produced highly similar tree topologies, support values are placed at the nodes in that order. The dash indicates branch support < 60. Red dots—calibration constraints used in the timetree analysis; blue dots—estimated divergence time. Sequences in bold with asterisks—karyotyped samples. Geographic map shows the distributional areas of three Nannospalax superspecies with sampling localities of all studied sequences (listed in Table 1, Tables S1 and S2, and presented in Figure S1). Source of the map: https://d-maps.com/(accessed on 29 March 2022).

Figure 3

ML phylogenetic tree inferred from MT-CYTB nucleotide comparison. Since both methods (ML and BI) produced highly similar tree topologies, support values are placed at the nodes in that order. The dash indicates branch support < 60. Red dots—calibration constraints used in the timetree analysis; blue dots—estimated divergence time. Sequences in bold with asterisks—karyotyped samples. Geographic map shows distributional areas of the three Nannospalax superspecies, with specified sampling localities of all studied sequences (listed in Table 1, Tables S1 and S2, and presented in Figure S1). Source of the map: https://d-maps.com/ (accessed on 29 March 2022).