Description of Malenchus sexlineatus n. sp., new records of three known species of Malenchus Andrassy, 1968 (Nematoda: Tylenchidae) and notes on amphidial aperture development

A new species, Malenchus sexlineatus n. sp., discovered from the Philippines, is described based on morphological and molecular data. The new species is unusual in the genus by having six lateral lines. Malenchus sexlineatus n. sp. is distinguished from M. williamsi, the only other species in the genus with six lateral lines (based on currently available SEM data), by a shorter body of 278 (270-288) vs 452 (425-495) mu m, shorter stylet (7.0 (6.2-7.5) vs 11-12 mu m), narrower annulations (0.8 (0.7-0.8) vs 1.2-1.6 mu m), lateral field comprising one elevated ridge in LM vs six well-separated incisures (resembling the lateral lines in Cephalenchus) in LM, the presence of S-shaped vs straight amphidial apertures, and vulval flaps absent or only one annuli long vs distinct. By having an exceptionally short body, M. sexlineatus n. sp. comes close to M. parvus, M. bryanti and M. acarayensis. However, there are significant differences in the lateral lines, annuli width and most morphometric ratios. Three known species, namely M. exiguus, M. nanellus and M. pachycephalus, all being first records and first representative from China, are characterised by morphological data. The new species was placed in a robustly supported clade containing two other Malenchus spp. and M. exiguus. Interestingly, M. pressulus was placed in a separate, unresolved phylogenetic position. However, the phylogenetic position of these clades could not be resolved within Tylenchidae. The shapes of the amphidial aperture and fovea within Malenchus are also compared and its possible developmental process is illustrated and discussed.