New insights into the evolutionary relationships of the genus Actus Baqri & Jairajpuri, 1974 (Nematoda, Mononchida) after the study of three Vietnamese species, with its updated taxonomy

Abstract

Three (one known and two new) species of the genus Actus, found in natural areas of Vietnam, are characterized, including morphological description, morphometrics, illustrations and molecular (18S, 28S rDNA) study. New data of Actus hagiangensis are provided, especially useful for comparative purposes. Actus bisexualis sp. n. is characterized by its 0.87–1.25 mm long body, lip region 20–23 μm wide, buccal cavity 25–28 μm, neck 255–320 μm long, female genital system diovarian, posterior (V = 62–65) and transverse vulva, tail conical (34–53 μm, c = 21–29, c’ = 1.6–2.2) and regularly curved, spicules 31 μm long, and 13 spaced ventromedian supplements. Actus similis sp. n. is distinguished by its 1.00–1.19 mm long body, lip region 21–22 μm wide, buccal cavity 24–25 μm long, neck 270–312 μm long, female genital system diovarian, posterior (V = 62–65) and transverse vulva, tail conical (40–46 μm, c = 24–28, c’ = 1.8–2.1) and regularly curved, and male absent. Morphologically, Actus is a very homogeneous taxon. Molecular analyses support its monophyly and its close evolutionary relationships with Sporonchulus, with Sporonchulinae being a monophyletic taxon too, but their kinship with other mononchid taxa cannot be established with certainty. The taxonomy of the genus is revised, with emended diagnosis, list of species, key to their identification and a compendium of their main morphometrics.