TAXONOMY OF LONGIDORID NEMATODES AND DICHOTOMOUS KEYS FOR THE IDENTIFICATION OF XIPHINEMA AND XIPHIDORUS SPECIES RECORDED IN BRAZIL

Ectoparasitic Longidoridae are globally an economically important family of nematodes that cause damage to an extensive range of crop plants by their feeding on plant root cells or transmitting viruses to a wide range of fruit and vegetable crops. Here, we provide an update review of Longidoridae taxonomy, including their basic morphology and the taxonomic characters used to distinguish the seven Longidoridae genera ( Australodorus, Longidorus, Longidoroides, Paralongidorus, Paraxiphidorus, Xiphidorus and Xiphinema ). In addition, dichotomous keys for the identification of Xiphidorus and Xiphinema species reported in Brazil are presented.


NEMATODES
The Phylum Nematoda is highly diverse in terms of species richness and one of the most abundant metazoan groups on earth (HUGOT et al., 2001).It is estimated that nematodes comprise nearly 90% of all multicellular organisms (Jairajpuri and Ahmad, 1992).Furthermore, LAMBSHEAD (1993) predicted the number of nematode species in marine habitats to be as high as one hundred million, although only 26,646 species have been currently described from all habitats (HUGOT et al., 2001).Nematodes are essentially aquatic organisms, the majority of which are microscopic in size (0.3-3.0 mm), living in a range of habitats, from oceans to the microscopic film of water surrounding soil particles (NORTON, 1978;DE LEY, 2000).Based on their different feeding habits, terrestrial and marine nematodes can be divided into different functional (trophic) groups (YEATES et al., 1993).
Economically, one of the most important functional nematode groups are the plant-parasitic nematodes that live in the soil or inside plant structures such as leaves, stems and mainly roots.Crop losses, in terms of reduced yield and quality, and management practices due to plant-parasitic nematodes were estimated annually at approximately 12% ( SASSER & FRECKMAN, 1987), corresponding to monetary losses to world agriculture of approximately US$100 billion.SASSER & FRECKMAN (1987) reported that the ten most economically damaging nematode genera were Meloidogyne, Pratylenchus, Heterodera, Ditylenchus, Globodera, Tylenchulus, Xiphinema, Radopholus, Rotylenchulus and Helicotylenchus.The majority of these plant-parasitic nematodes belong to the order Tylenchida, except Xiphinema (Longidoridae) that is a member of Dorylaimida.
The main focus of this review is the longidorid group of nematodes, especially the genera Xiphinema DOI: 10.1590/1808-1657v73p1312006 and Xiphidorus that cause damage to crop plants by their direct feeding on root tips, resulting typically in stunted plant growth with concomitant yield reduction.A few Xiphinema species are also known as vectors of nepoviruses (BROWN et al., 2004) and some of which (X. americanum sensu stricto, X. bricolense, X. californicum and X. rivesi) are quarantine pests (http:/ /www.eppo.org/QUARANTINE/lists.htm).

Classification
Reviewing the systematics of Longidoridae, HUNT (1993) proposed an intermediate classification between the inflationary concept of KHAN et al. (1978) and the more conservative schemes of both LUC & DOUCET (1984) and COOMANS (1985).Later, based on phylogenetic relationships of morphological characters, COOMANS (1996) suggested two possible classifications; the first included a Tribe subdivision and the second scheme was similar to that proposed by HUNT (1993) but updated to include Paraxiphidorus.This study has adopted the classification of Hunt (1993)

Taxonomy of Longidoridae
Longidorids are relatively large nematodes (2 to 12 mm in length), with a slender body and have a long, hollow feeding spear (60-250 µm in length) that differentiates Longidoridae from other Dorylaimid groups.The spear is comprised anteriorly by an odontostyle, that punctures the root tip and enables the nematode to feed within root cells whilst remaining exterior to the root, and posteriorly by an odontophore (ARIAS & BRAVO, 1997).The oesophagus consists of a long, narrow anterior tube connecting the spear with a cylindrical bulb that provides the pumping action used to withdraw plant cell contents.(Fig. 1) (BROWN et al., 1995).The bulb contains three large gland cells, one dorsal and two subventral.
Females have various arrangements of possible reproductive systems (Fig. 2): amphidelphic (two genital branches, one running anteriorly and the other posteriorly), monodelphic (one posterior genital branch) or pseudomonodelphic (one posterior functional genital branch whilst the anterior branch is reduced, atrophied and non-functional) (COHN & SHER, 1972;COOMANS et al., 2001).The majority of species have an assumed parthenogenetic mode of reproduction since males are either not known or rare.However, amphimictic reproduction occurs with species where males have been recorded (COOMANS et al., 2001).When present, males have curved spicules with lateral accessory pieces (crura) and ventromedian supplements.Tail morphology is similar in both sexes.Usually there are four juvenile stages, but in a few species there are only three (HALBRENDT & BROWN, 1992;ROBBINS et al., 1996).Juveniles have a similar morphology to that of adults (ARIAS & BRAVO, 1997).

Xiphinema and Xiphidorus taxonomy
Xiphinema and Xiphidorus are the most common longidorid nematodes present in Latin America (see section 4).Identification of Xiphinema and Xiphidorus species is based mostly on female morphology and morphometrics.The most useful taxonomic characters used to separate species are considered to be body length, habitus, shape and size of lip region, shape and size of amphid fovea, spear (odontostyle and/or odontophore length), length and shape of tail, vulva position, characteristics of the female genital tract and uterine differentiation (ARIAS & BRAVO, 1997;BROWN, 1997).Xiphinema species have a characteristic flanged odontophore, forked junction of the odontostyle and odontophore, posteriorly located spear guiding apparatus near the odontostyle base, amphid funnel to stirrup shape with aperture slit-like and dorsal gland nucleus close to dorsal gland opening (Fig. 1) (Table 1) (LUC & DOUCET, 1984).
A polytomous key was erected for the identification of Xiphinema based on their comparatively large morphological diversity and relative ease of distinguishing morphological and morphometric characteristics for the majority of species (LOOF & LUC, 1990, 1993;LOOF et al., 1996).However, species comprising the X. americanum-group were excluded from this polytomous key as several Recently, revised polytomous and dichotomous keys for the identification of 49 (two are now species inquirendae) X. americanum-group species were published (LAMBERTI et al., 2004), based on primary quantitative taxonomic characters, namely: odontostyle and tail length, and the ratios c' (tail length/body width at anus), V (distance from head end to vulva/body length) ( SIDDIQI, 2000b), a (body length/maximum body length) and c (body length/ tail length) proposed by de MAN (1884).

Xiphidorus
The Longidoridae genus, Xiphidorus, comprises nematodes with the following taxonomic characteristics: flanged odontophore, a forked junction of the odontostyle and odontophore, posteriorly located spear guiding apparatus near the odontostyle base, amphid pouch shape with an aperture as a small transverse slit, dorsal gland nucleus some distance from dorsal gland opening and the subventral gland nuclei more developed than the dorsal gland nucleus (Fig. 1).Xiphidorus combines taxonomic characteristics of other longidorids.The spear Fig. 3 -Taxonomic characters used to distinguish Longidoridae genera.See Table 1 1) (MONTEIRO, 1976;LUC & DOUCET, 1984).
Nematodes of this genus are indigenous to Latin America (COOMANS, 1985;DOUCET et al., 1998) and thus have a more restricted geographical distribution than the closely related Xiphinema (COOMANS et al., 2001).

ECONOMIC IMPORTANCE OF LONGIDORIDS
As noted in section 1, longidorid nematodes cause damage to an extensive range of crop plants by their direct feeding on plant root cells.However, a few species are also capable of transmitting viruses, leading to diseases in a wide range of fruit and vegetable crops (BROWN et al., 1995;BROWN et al., 2004;TAYLOR & BROWN, 1997) and are of potentially greater economic importance.
The economic importance of Xiphidorus with respect to crop damage is unknown.However, histopathological studies under controlled conditions indicated that X. minor fed on the root tips of rice and tomato (LEONE et al., 1999) resulting in swollen tips, typical of damage by longidorid nematodes ( TAYLOR & BROWN, 1997).

Virus transmission
The natural transmission of a nepovirus by Longidoridae was first demonstrated by HEWITT et al. (1958) who reported X. index as the natural vector of grapevine fanleaf virus in vineyards in California.Currently, eight Longidorus, one Paralongidorus and ten Xiphinema species are vectors of 12 viruses belonging to the genus Nepovirus (Table 2) (BROWN et al., 2004).
Most of the previous research on longidorids has occurred in North America and Europe, with few investigations having been done in Latin America, Asia, or Africa.In North America, four viruses (cherry rosette disease, peach rosette mosaic, tobacco ringspot and tomato black ring) transmitted by species belonging to the X. americanum-group cause damage to a wide range of fruit and vegetable crops (TAYLOR & BROWN, 1997).

Direct damage
Despite the many studies on the taxonomy, systematics and the geographical distribution of longidorid nematodes, there is a paucity of experimental data concerning the direct damage (cf.indirect damage due to virus transmission) caused by indivi dual species on particular hosts (TAYLOR & BROWN, 1997).However, the pathogenicity of Xiphinema species has been demonstrated for a few host plants.For example, X. longicaudatum severely depressed the growth of eggplant (Solanum melongena) in Africa (Fig. 4) in a "screen house" (LAMBERTI et al., 1992).Plants infected with this nematode were stunted and had a reduced root system.Also, X. ifacolum suppressed the plant growth of okra, pepper, rice and tomato ( LAMBERTI et al., 1987a;LAMBERTI et al., 1992).
The taxonomy of X. parthenus sensu MONTEIRO et al. (1981) and X. yepesara sensu MONTEIRO (1976) is controversial. DECRAEMER et al. (1996) queried the classification and suggested the replacement of both species with two sub-species, namely X. yepesara parthenus and X. yepesara yepesara., CHAVES et al. (1999) rejected DECRAEMER et al. (1996) and suggested synonymization of both species to X. yepesara.However, recently, morphometric and molecular data suggested that X. parthenus and X. yepesara are distinct taxonomic species contrary to their previous subspecies status and synonymization.Thus the retention of the original species proposed by MONTEIRO (1976) and MONTEIRO et al. (1981) is recommended (OLIVEIRA et al., 2004).

Subsequently
. KEY TO IDENTIFICATION OF XIPHINEMA SPECIES RECORDED IN BRAZIL.
A dichotomous key was prepared based on the appropriate morphological and morphometric characteristics of female Xiphinema species recorded in Brazil (OLIVEIRA et al., 2003).The characters used were according to LOOF & LUC (1990), LAMBERTI et al. (2000) and COOMANS et al. (2001).Xiphinema species feed on the root tips or other parts of young, actively growing roots ( TAYLOR & BROWN, 1997).For example, X. diversicaudatum feeds at the root tips, causing prominent and subterminal swellings in roots of rose, strawberry, celery, several crop plants and weeds ( TAYLOR & BROWN, 1997 and references therein).Also, X. bakeri feeding at the root tips of Pseudotsuga menziesii seedlings caused darkening, swelling and cessation of root growth (SUTHERLAND & DUNN, 1970).However, Xiphinema spp.are not exclusively root tip feeders as X. brevicolle and X. index were observed to feed along the seedling roots of Bidens tripartite, Urtica urens and Vitis vinifera causing darkening of roots and cortex breakdown (COHN, 1970).

The family Longidoridae in Latin America
Of the seven Longidoridae genera, only species belonging to Australodorus, Longidoroides, Longidorus, Paraxiphidorus, Xiphidorus and Xiphinema have been reported from Latin America (DOUCET et al., 1998, COOMANS et al., 2004).From the 53 valid longidorid species reported by DOUCET et al. (1998), 42 were Xiphinema, 22 of which, (X.basiri, Fig. 1 -Basic morphology of the anterior region of Xiphidorus and Xiphinema. species of this group are only distinguished by minor morphometric or morphological differences ( LOOF & LUC , 1990; BROWN & HALBRENDT, 1997).Regional polytomous keys were published (LAMBERTI et al., 2000) as a practical means for identifying the 51 (now 50 species after taxonomic revision by LAMBERTI et al., 2004 and description of X. parasimile by BARSI & LAMBERTI, 2004) putative species composing of the X. americanum group, although reservations about the reliability of the codes used in this key have subsequently been presented (LUC & BAUJARD, 2001).