Apatania zonella

Salokannel, Juha, Lee, Kyung Min, Rinne, Aki & Mutanen, Marko, 2021, ddRAD Sequencing Sheds Light on Low Interspecific and High Intraspecific mtDNA Divergences in Two Groups of Caddisflies, Insect Systematics and Diversity (AIFB) 5 (5), pp. 1-10 : 8

Apatania zonella Group

The present DNA barcoding results are similar to the study by Salokannel et al. (2010), even if these studies had only one common sample (ME071): (i) despite relatively small divergences, each valid species of A. zonella group has its own cluster and appear monophyletic; (ii) A. nr. hispida specimens have identical DNA barcode with A. hispida ; and (iii) specimens identified as A. zonella show more diversity than the other taxa of the group ( Fig. 2A and Table 2). Also, the ddRAD-seq results are mainly in line with the mtCOI results, supporting the existing taxonomic notion of the presently valid species.

The specimens of A. nr. hispida differentiate from A. hispida in the ddRAD-seq data, but not as significantly as the other analyzed species differentiate from each other. They also do not form reciprocally monophyletic entities. The combination of different morphology, identical mtCOI and slightly deviated ddRAD-seq suggests that A. nr. hispida and A. hispida are very closely related, but genetically distinct taxa. Because of parthenogenetic nature of these taxa (no males are known), their taxonomic status cannot be assessed under the biological species concept. A possibility remains that they represent relatively old, diverged asexual strains. Taxonomic delineation of asexual taxa is often difficult and inherently arbitrary. Considering that these two taxa appear as fully asexual, it is surprising that the D-statistics we conducted suggested introgression to have happened between them. Two possible scenarios might explain this situation. First, signs of introgression may originate time before they turned asexual. Our phylogeny suggests asexuality being a derived feature in this group. Second, A. hispida and A. nr. hispida might represent two independent shifts from sexual to asexual mode of reproduction, as has been demonstrated in a similar system in Dahlica moths ( Elzinga et al. 2013). A question follows: what could this sexual ‘stem’ species then be? While not sampled here, an excellent candidate could be the sexual A. majuscula . Its DNA barcodes available in the BOLD systems (incl. our own data) reveals it being very closely related to the A. hispida complex, with barcodes being partially phylogenetically nested within the A. hispida —A. nr. hispida cluster. Its female genitalia also show close resemblance to those of A. hispida and A. nr. hispida . While we unfortunately did not include A. majuscula in our ddRAD sampling, we find the scenario of A. hispida and A. nr. hispida representing two asexual strains of A. majuscula appealing. Further studies are needed to address this question.

The unidentified specimen JSlk-2016R003A with exceptional morphology clusters with A. forsslundi in mtCOI, but ddRAD suggests admixture of between A. zonella and A. auricula . This is also biologically possible as both species, especially A. zonella , are known to produce males regularly in its Nordic populations. Altogether, the DNA results suggest hybridization between two or more species. Also, the hybridization may have occurred already in the past and the hybrid line might have survived, supported by parthenogenesis. Such hybrid lines could also explain other cases of morphologically intermediate or aberrant specimens that have caused issues for identifiers over the decades ( Solem 1985, personal observations).

Figures

Fig. 2. Maximum likelihood trees based on mitochondrial COI sequences of Apatania (A) and Limnephilus (B). The bootstrap values are indicated near the branches (only shown for nodes supported with more than 50% of BS).