Abstract
Phylogenetic framework for the closely related Ancylodiscoidinae and Ancyrocephalinae subfamilies remains contentious. As this issue was never studied using a large molecular marker, we sequenced the first two Ancylodiscoidinae mitogenomes: Thaparocleidus asoti and Thaparocleidus varicus. Both mitogenomes had two non-coding regions (NCRs) that contained a number of repetitive hairpin-forming elements (RHE). Due to these, the mitogenome of T. asoti (16,074 bp) is the longest among the Monogenea; especially large is its major NCR, with 3500 bp, approximately 1500 bp of which could not be sequenced (thus, the total mitogenome size is ≈ 17,600 bp). Although RHEs have been identified in other monopisthocotyleans, they appear to be independently derived in different taxa. The presence of RHEs may have contributed to the high gene order rearrangement rate observed in the two mitogenomes, including the first report of a transposition of rRNA genes within the Neodermata. Phylogenetic analyses using mitogenomic dataset produced Dactylogyrinae embedded within the Ancyrocephalinae (paraphyly), whereas Ancylodiscoidinae formed a sister-group with them. This was also supported by the gene order analysis. 28S rDNA dataset produced polyphyletic Dactylogyridae and Ancyrocephalinae. The phylogeny of the two subfamilies shall have to be further evaluated with more data.
Highlights
Species from the genus Thaparocleidus (Ancylodiscoidinae or Ancylodiscoididae, see below) are common monogenean parasites found in catfishes, but their phylogeny remains debated
As all of the above studies were based on morphological data, several studies later attempted to resolve the phylogenetic relationships within the family Dactylogyridae using molecular data: 18S rDNA [7], 28S rDNA [8], and a combination of 18S, 28S and 16S rDNA [9]
With regards to the three Dactylogyridae subfamilies, the results suggest a closer relationship between Ancyrocephalinae and Dactylogyrinae than between Ancylodiscoidinae and Ancyrocephalinae subfamilies, which contradicts some of the morphology-based hypotheses that grouped the species of Ancylodiscoidinae within the Ancyrocephalid(n)aefamily [1,4]
Summary
Species from the genus Thaparocleidus (Ancylodiscoidinae or Ancylodiscoididae, see below) are common monogenean parasites found in catfishes, but their phylogeny remains debated. As all of the above studies were based on morphological data, several studies later attempted to resolve the phylogenetic relationships within the family Dactylogyridae using molecular data: 18S rDNA [7], 28S rDNA [8], and a combination of 18S, 28S and 16S rDNA [9] In these studies, predominantly marine Ancyrocephalinae species (with a few freshwater outliers, see clade B in [8]) constantly clustered with the Dactylogyrinae and Pseudodactylogyrinae species (M-clade ). M-clade formed a sister-clade with AA-clade (but with low support), whereas freshwater Ancyrocephalinae were basal to them, i.e., formed a sister-clade with the former two groups Both morphological and molecular studies failed to obtain monophyletic Ancyrocephalinae, and the relationship of Ancylodiscoidinae and Ancyrocephalinae remains unresolved
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