Phylogeny, diversity, and species delimitation in some species of the Xiphinema americanum-group complex (Nematoda: Longidoridae), as inferred from nuclear and mitochondrial DNA sequences and morphology

Abstract

During nematode surveys in southern Spain and Italy 14 populations of Xiphinema species tentatively identified as Xiphinema americanum-group were detected. Morphological and morphometrical studies identified three new species and six known Xiphinema americanumgroup species, viz.: Xiphinema parabrevicolle n. sp., Xiphinema parapachydermum n. sp., Xiphinema paratenuicutis n. sp., Xiphinema duriense, Xiphinema incertum, Xiphinema opisthohysterum, Xiphinema pachtaicum, Xiphinema rivesi, and Xiphinema santos. The Xiphinema americanum-group is the most difficult Xiphinema species group for diagnosis since the morphology is very conservative and morphometric characters often overlap. This group includes vectors of several important plant pathogenic viruses that cause significant damage to a wide range of agricultural crops. Molecular characterisation of these species using D2-D3 expansion regions of 28S rRNA, 18S rRNA, ITS1-rRNA and the protein-coding mitochondrial gene, cytochrome oxidase c subunit 1 was carried out and maximum likelihood and Bayesian inference analysis were used to reconstruct phylogenetic relationships among these species and with other Xiphinema americanum-group species.

Morphological and molecular characterization of Xiphinema species from Shenzhen, China

“…This group may contain many cryptic species that are morphologically indistinguishable but may be phylogenetically distant to one another (Gutiérrez-Gutiérrez et al 2010, 2012Barsi & De Luca 2008;Oliveira et al 2005Oliveira et al , 2006Wu et al 2007;Ye et al 2004). In the present study, analysis of morphology and morphometrics indicated that fi ve studied populations (SZX1306 -SZX1310) were very similar to some members belonging to X. americanum-group.…”

“…Four studied populations (KP793036 -KP793039) of X. (Lamberti & Golden, 1984), from X. incognitum by having a smaller lip (10.0 -11.2 vs 11.0 -13.0 μm), from X. diffusum by having a smaller lip (10.0 -11.2 vs 11.0 -13.0 μm), a more posterior guide ring (69.3 -88.9 vs 60.0 -64.0 μm), from X. taylori by having a shorter (1644 -2106 vs 2100 -2500 μm), a higher c' value (0.9 -1.4 vs 0.7 -0.8) and a longer tail (23.9 -33.2 vs 19.5 -22.0 μm), from X. parabrevicolle by having a higher c' value (0.9 -1.4 vs 0.7 -0.8), a smaller lip (10.0 -11.2 vs 12.5 -14.0 μm) (Gutiérrez-Gutiérrez et al 2012), from X. brevicolle by having a shorter stylet (137 -143 vs 144 -173 μm) (Lordello & Costa 1961;Lamberti et al 1991;Luc et al 1998), a lower b value (5.3 -7.7 vs 7.0 -10.5) (Lordello & Costa 1961). Nevertheless, these hunaniense from China had 100 % identities (647/647=100 %) based on alignments of the sequences of 18S rDNA.…”

“…There are approximately 260 nominal species in the genus to date (Gutiérrez-Gutiérrez et al, 2012). They are typically divided into two groups, namely X. americanum-group with about 50 species and non-X.…”

“…However, species belonging to Xiphinema americanum-group show conserved morphology and overlapping morphometrics Gutiérrez-Gutiérrez et al, 2012). Thus, DNA-based approaches including ribosomal DNA such as the 18S, D2-D3 expansion segments of 28S, ITS regions, and mitochondrial DNA have been employed for the molecular characterization and reconstruction of phylogenetic relationships within Xiphinema (Oliveira et al, 2004;Ye et al, 2004;He et al, 2005;Lazarova et al, 2006;Wu et al, 2007;Gutiérrez-Gutiérrez et al, 2012.…”

“…Thus, DNA-based approaches including ribosomal DNA such as the 18S, D2-D3 expansion segments of 28S, ITS regions, and mitochondrial DNA have been employed for the molecular characterization and reconstruction of phylogenetic relationships within Xiphinema (Oliveira et al, 2004;Ye et al, 2004;He et al, 2005;Lazarova et al, 2006;Wu et al, 2007;Gutiérrez-Gutiérrez et al, 2012. Some species of the genus Xiphinema are distributed worldwide, whereas others have limited distribution (Coomans, 1996;Coomans et al, 2001;Gutiérrez-Gutiérrez et al, 2012). So far, 14 species of Xiphinema (X. americanum Cobb, 1913, X. brasiliense Lordello, 1951, X. brevicolle Lordello & Costa, 1961, X. diffusum Lamberti & Blève-Zacheo, 1979, X. elongatum Schuurmans Stekhoven & Teunissen, 1938, X. hunaniense , X. imitator Heyns, 1965, X. incognitum Lamberti & Blève-Zacheo, 1979, X. insigne Loos, 1949, X. luci Lamberti & Bleve-Zacheo, 1979; X. oxycaudatum Lamberti & Blève-Zacheo, 1979, X. radicicola Goodey, 1936, X. taylori Lamberti, Ciancio, Agostinelli & Coiro, 1992, X. thornei Lamberti & Golden, 1986) were reported in China (Luo et al, 2001;Pan et al, 2000;Teng et al, 2013;Wang et al, 1996;Wu, 2007;Xu et al, 1995;Zheng & Brown, 1999).…”