First molecular study of Metarhabditis costai (Martins, 1985) Sudhaus, 2011 (Rhabditida, Rhabditidae) from India and its taxonomic implications

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Summary Insect-associated nematodes are an ecologically important yet understudied component of soil biodiversity, particularly in tropical and subtropical agroecosystems. Here, we report the first molecular characterisation and comparative analysis of Metarhabditis costai from India by combining morphological and molecular approaches. The studied population was isolated from the rhizosphere of Sorghum bicolor L. in Jaipur, Rajasthan, using the Galleria mellonella baiting method. The studied population of the species is characterised by a body length of 0.87-1.25 mm length in females; a finely straited cuticle; lateral fields with five incisures; a lip region continuous with the adjacent body, small, oval amphidial openings; and a rhabditoid stoma with a buccal tube approximately four times longer than wide. The pharynx possesses a slightly swollen metacorpus, with nerve ring and excretory pore located at level of the isthmus. Females are didelphic-amphidelphic, with a slightly protruding vulva, a rectum measuring 0.7-1.4 times the anal body width, a conical-elongate tail with an acute terminus (84-114 μ m, c = 9.1-14.2, c′ = 3.8-4.5), and phasmids located at 26-42% of tail length. Males are monorchic, with spicules 25-38 μ m long, with a truncated manubrium, and a straight lamina with a ventrally bent tip; gubernaculum 11-18 μ m long; bursa leptoderan with eight genital papillae (1 + 1 + 1/3 + 2) plus a fine phasmid posterior to GP8, and a conical tail (20-32 μ m, c = 21-34, c′ = 0.8-1.5) with a filiform extension beyond the bursa. Compared with the type population, intraspecies variations were observed in stoma length, distance from anterior end to the nerve ring and pharynx length in females. In males, variations were observed in pharynx length, maximum body diam., tail length, gubernaculum and total body length. Molecular phylogenetic analyses based on 18S and ITS rDNA sequences were conducted for the first time for this species. In Bayesian phylogenetic reconstructions, the newly generated sequences from the Indian population clustered with available sequences of the genus in both datasets, forming well-supported clades.

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