T. T. Tsay

Differentiation of Hemicriconemoides mangiferae and H. litchi (Nematoda: Criconematina) based on morphometrics and nuclear ribosomal DNA sequences

Hemicriconemoides mangiferae and H. litchi, which have been considered as junior synonyms of H. strictathecatus, are morphologically similar, making identification difficult. In the present study, six populations of Hemicriconemoides spp. were collected from mango and litchi in Taiwan and were differentiated based on morphometrics and molecular characteristics. By comparing morphometric data from females and males, three populations collected from mango were identified as H. mangiferae and another three populations from litchi were identified as H. litchi. According to SEM, both of the H. mangiferae and H. litchi males had an areolated lateral field with four incisures; caudal alae were not found. The internal transcribed spacer (ITS) sequences of ribosomal DNA (rDNA) from these nematode populations were sequenced and aligned. The sequence identity of the ITS region was 89-95% between H. mangiferae and H. litchi populations, suggesting rDNA sequence divergence could be a taxonomic aid for species differentiation. Since H. mangiferae and H. litchi are closely related to H. strictathecatus, more rDNA sequence information could be valuable to deal with molecular phylogenetic relationships of these species in depth.

Differentiation of the Xiphinema americanum -group nematodes X. brevicollum , X. incognitum , X. diffusum and X. oxycaudatum in Taiwan by morphometrics and nuclear ribosomal DNA sequences

Morphometrics and molecular variability of the X. americanum-group collected in Taiwan were studied. Morphometric data, derived from the females and their developing juveniles, revealed that the 27 populations comprised four taxa: X. brevicollum, X. incognitum, X. diffusum and X. oxycaudatum, the last species being found to have only three juvenile stages. Further identifications were conducted by analysing the nucleotide sequences of the first internal transcribed spacer (ITS-1), 5.8S gene and second internal transcribed spacer (ITS-2) of ribosomal DNA (rDNA). Pairwise comparisons revealed the sequence differences among these taxa ranging from 3.5 to 21.8% for the ITS-1, 2.9 to 28.6% for the ITS-2 and 0 to 3.1% for the 5.8S. Little or no intraspecific variation was detected in the four species for which multiple populations from different geographical regions or hosts were sequenced. Although X. incognitum is morphometrically closer to X. diffusum than it is to X. brevicollum in mean body length, total stylet length, and percentage position of vulva, there is a higher degree of genetic similarity between X. incognitum and X. brevicollum.