Combining morphological and molecular data to classify Laxus sakihariiae sp. n., a new stilbonematine nematode (Nematoda: Desmodoridae) from the coast of Sesoko Island, Japan

in Nematology
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Because of their relatively simple body plan, the number of morphological characters used to differentiate between closely related nematode genera is often limited. In addition, boundaries among genera sometimes become blurred due to the appearance of new trait combinations as more new species are described. Molecular phylogenetic analyses can address the shortcomings of morphological taxonomy by clarifying relationships among closely related species and genera and can help identify which morphological characters are taxonomically informative. Here, we describe Laxus sakihariiae sp. n. from shallow subtidal sands on Sesoko Island in the Okinawa prefecture, investigate phylogenetic relationships with other stilbonematine species and genera based on SSU rDNA sequences, and provide the first LSU rDNA sequence for the subfamily. The new species can be easily distinguished from all other species of the genus by the presence in the male of subventral and ventral rows of stout and spine-like setae in the pre- and postcloacal regions. This feature suggests affinities with the closely related genus Leptonemella, although the SSU consensus tree clearly shows that the new species forms a monophyletic clade together with the other Laxus species for which sequences are available. The structure of the cephalic capsule in L. sakihariiae sp. n., which consists of a block layer between the median and basal zones of the cephalic cuticle, is consistent with the placement of this species. This trait is not currently used as a diagnostic feature, but our finding suggests that the structure of the cephalic capsule may be taxonomically useful for differentiating between some stilbonematine genera.

Combining morphological and molecular data to classify Laxus sakihariiae sp. n., a new stilbonematine nematode (Nematoda: Desmodoridae) from the coast of Sesoko Island, Japan

in Nematology



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    Laxus sakihariiae sp. n. A: Anterior body region of male; B: Surface view of male cephalic region; C: Optical section of male cephalic region; D: Posterior body region of male; E: Posterior body region of female. (Scale bar: A = 50 μm; B, C = 40 μm; D = 65 μm; E = 58 μm.)

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    Laxus sakihariiae sp. n. A: Cephalic region of female; B: Female reproductive system; C: Entire male. (Scale bar: A = 40 μm; B = 215 μm; C = 355 μm.)

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    Laxus sakihariiae sp. n. Light micrographs. A: Subventral glandular sensory organs (lateral view); B: Sublateral sensory organs (apical view). (Scale bar = 20 μm.)

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    Laxus sakihariiae sp. n. Scanning electron micrographs. A: Entire juvenile; B, C: juvenile cephalic region; D, E: Male cephalic region; F: Apical view of female cephalic region; G: Mouth opening of male. (Scale bar: A = 200 μm; B, D = 15 μm; C = 16 μm; E = 12 μm; F = 9 μm; G = 4 μm.)

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    Laxus sakihariiae sp. n. Scanning electron micrographs. A: Ventral view of male posterior body region; B: Apical view of tail tip showing outlets of caudal glands and postcloacal conical setae; C: Ventral view of male precloacal setae and spines; D: Lateral view of female posterior body region. (Scale bar: A = 40 μm; B = 9 μm; C = 13 μm; D = 21 μm.)

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    Bayesian tree inferred from SSU sequences under the general time-reversible (GTR) + gamma distribution (G) model. Only SSU sequences longer than 1400 bp were included. Posterior probabilities (left) and bootstrap values (right) from the ML analyses greater than or equal to 50% are given on appropriate clades. The scale stands for substitutions per site.

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