Xiphinema is a very large, diverse and complex group of nematode
species. Morphological identification in this group is complex due to
overlapping morphometrics and the sharing of many morphological characters.
Reliable diagnostic methods are needed to enable distinguishing between
species for management, quarantine or regulatory purposes.
The genus may
be divided into the americnum and non-americanum lineages.
The americanum lineage consists of around 55 nominal species and cryptic
species, many of which are morphologically indistinguishable, and the
non-americanum groups of more than 200 more-or-less distinguishable species.
General Characteristics of the Genus:
is long, connected to lining of
cheilostome by a cuticular membrane that becomes variously folded, with
refractory qualities - the "guiding ring."
Species in the genus Xiphinema are relatively large nematodes, 2 to 3 mm in length.
The genus is characterized by the presence of a very long odontostyle
or spear (stylet).
The spear and its extension are approximately 150 µm or more in length.
A flanged odontophore is characteristic of Xiphinema.
Xiphinema has a 2-part
esophagus typical of most
with a slender anterior part and swollen glandular and muscular posterior bulb.
The cross-sectional configuration of the stylet is cylindrical and the
cross-section of the odontophore is tri-radiate as in the esophagus. The
esophagus has a tri-questrous lumen.
have paired pre-anal papillae and a ventromedian row of supplementary
papillae. The number and arrangement of these papillae are of importance
in distinguishing species.
Males have paired spicules, but
males have no bursa.
Females of Xiphinema have one or two
ovaries. The ovaries,
when both occur, are opposed and reflexed.
Identification of Species of Xiphinema
Loof and Luc (1990) recognized 172 valid species of Xiphinema by
that time and more have been added since. They developed a polytomous key which
was split into eight groups on the basis of development of the anterior female
genital branch, uterine differentiation and tail shape.
The groups are distinguished by the following criteria:
Code A: type of female genital
tract, the comparative development and
structure of the two female genital branches.
Code A1: the
anterior genital branch is absent: the ovojector is generally symmetrical but
may also be somewhat asymmetrical anteriorly, the vagina is directed slightly
backward (X. brasiliense). The vulva
is usually anterior (C = Ã‚Â± 30).
Code A2: the
anterior uterus may be as long as the posterior one (X.
surinamense, X. filicaudatum, X. dimidiatum) but is usually reduced in
length and width. The oviduct is greatly reduced to a mass of cells in which
generally no lumen or structure can be recognized. The anterior ovary is absent.
Code A3: all
the components of the anterior branch are present, but greatly reduced in
comparison to those of the posterior branch).
there are two fully, equally developed genital branches.
X. brasiliense (A),
X. radicicola (B)
Code A2: X.
longicaudatum (C,D), X. krugi
(E,F), X. costaricense (G,H),
X. filicaudatum (I)
Code A3: X.
Drawings from Loof and Luc (1990)
Code B: Uterine differentiation.
Code B1: the
Z-organ which lies at the junction of the pars dilatata uteri and the distal,
more or less straight, part of the uterus is rather well separated from these
sections and has a heavy wall composed of strong circular muscles; the lumen has
a conspicuous refringent lining which is regularly longitudinally folded; and a
relatively low number (3-5) of yellowish, often angular, apophyses protrude into
Code B2: the
pseudo-Z-organ is found in the same place as the Z-organ but is less well
structures; the wall is thinner, the muscles are weaker or almost absent; the
lining is thin, its longitudinal folding is often difficult to observe. The
apophyses are rarely angular (X. pini)
and vary in shape: they appear as composite bodies with a clear central globular
part surrounded by small, less translucent globules (e.g.
X. diversicaudatum); simple globular bodies (X.
marsupilami, X. basiri) of
uterine spines are present either all over the length of the tubular part of the
uterus (X. spinuterus) or only in the
part close to the uterine pouch (X.
xenovariabile). They are of variable length and density, but are always
directed away from the vagina. In some species both spines and pseudo-Z organ
are present (e.g. X. malagasi).
Code B1: X.
ifacolum (A), X. ebriense (B)
Code B2: X. pini
X. turcicum (E),
X. basiria (F),
X. ingens (G),
X. marsupilami (H)
Code B2/3: X.
Code B3: X.
Drawings from Loof and Luc, 1990
Notet: (1) uterine differentiation appears linked to the presence of two
complete female genital branches, the only exception being
X. dimidiatum in which the anterior
branch is reduced and the posterior one contains uterine spines; and (2) the
presence of a Z- or pseudo-Z-organ is more frequent among species having
Code C: Female tail shape is one
of the most variable characters in the genus. The various tail shapes can be
long filiform to conical shape; the
shortest tails (C4) may be somewhat digitate
Code C1: X.
Code C2: X.
bergeri (B), X. douceti (C),
X. nigeriense (D),
X. insigne (E),
X. attarodorum (F),
X. clavicaudatum (G)
Code C2/3: X.
Code C3: X.
ifacolum (J), X. longidoroides
Code C3/4: X. coxi
(L), X. setariae
Code C4: X. basiri
Drawings from Loof and Luc, 1990
short, more or less rounded to perfectly hemispherical; sometimes
provided with a short peg (C5), or a long one (C8)
Code C5: X.
mammallatum (A), X. thorneanum
(B), X. brasiliense
Code C6A: X.
melitense (D), (E.F)
Code C6B: X.
bacaniboia (E), X. surinamense
Code C7A: X.
Code C7B: X.
porosum (H), X. hygrophilum (I)
Code C8: X.
Tail shapes are characterized as:
Code C1: Tail long, attenuated or
filiform (c' over 7.5)
Code C2: Tail long (c' between 2.5 and
7.5), conical or with clavate terminus
Code C3: Tail regularly short conical
(c' at most 2.5), i.e. tapering uniformly to narrowly rounded or acute terminus;
or slightly subdigitate
Code C4: Tail short conical (c' at most
2.5), distinctly digitate
Code C5: Tail conical to hemispherical
with a terminal peg, mucro or bulge
Code C6: Tail broadly convex-conoid,
i.e. tapering to broadly rounded terminus with main curvature on dorsal contour,
the ventral one being almost straight
Code C7: Tail regularly hemispherical,
i.e. ventral and dorsal curvatures equal
Code C8: Tail conical rounded, with
very long clavate peg
Code D: coefficient c' – ratio of
tail length to anal body diameter
Code D2: 5.1-7.5
Code D3: 2.6-5.0
Code D4: 1.6-2.5
Code D5: 1.1-1.5
Code D6: <1.0
Code E: coefficient V – position
of vulva (% body length)
Code E1: <30
Code E2: 30-34
Code E3: 35-39
Code E4: 40-44
Code E5: 45-49
Code F: L (body length)
Code F1: <1.5 mm
Code F2: 1.5-2.4 mm
Code F3: 2.5-3.4 mm
Code F4: 3.5-4.4 mm
Code F5: >4.5 mm.
Code G: s spear length
Code H: Shape of anterior part of
body (note, body must be in lateral position to avoid confusing amphid apertures
Code H1: no constriction between head
region and body, continuous outline
Code H2: intermediate constriction
between head and body
Code H3: very conspicuous constriction
between head and body
Code H1: X.
hygrophylum (A), X. stenocephalum
Code H2: X.
elongatum (C,D), X. coxi (E,F),
X. insigne (G),
X. ingens (H)
Code H3: X.
conurum (I), X. pachydermum (J)
Code I: (female body posture when
Code I1: Body straight or nearly so
Code I2: Body weakly curved
Code I3: Body hook-shaped, or C- or
Code I4: Body spiral-shaped
Code J: (tail shape of 4th
Same divisions as for Code C
Code K: (tail shape of 1st
Code L: Presence or absence of
males (often correlated with presence or absence of sperm in the female genital
Code L1: Unknown or very rare (female
generally devoid of sperm)
Code L2: Abundant (female generally
provided with sperm).
Using the Polytomous Key of Morphotypes
The characteristics corresponding to each letter of the code are observed,
measured or calculated; each of them is allocated the corresponding figure
according to the preceding paragraph (the 'new code').These figures are put
on a strip of paper, so that each corresponds to the column of each letter. For
example, in the following combination:
On the list under Group 5, the strip of paper is moved from the top of the
lattice until a species is found in which the figures correspond to those
on the strip, in this case X. natalense:
For each column, only one identical figure is needed; in the example
chosen, for character G (spear length) the code is 2/3 because the spear length
range of the species (197-204 µm) overlaps the crieria for Codes G2 and
G3. If there is no identity for all the letters in any horizontal line, the
specimens being identified may represent an undescribed species. Here
molecular tools will be important to confirm differences from other species.
As emphasized by Loof and Luc (1990), the eight morphotype
groups have no taxonomic value and do not infer phylogenetic relationships.
Rather, they assemble the known species into groups that facilitate further
identification based on additional characters.
Refer to Loof and Luc, 1990 for a complete list of species in each
morphotype group as of 1990. Others have been charecterized since then,
codes for some are provided in Loof and Luc (1993) and Loof et al. (1996).
Members of the genus occur worldwide.
Many species ar important parasites of plants, including many agricultural
Ectoparasites; feed along root, including root
tips. Species of Xiphinema feed at root tips and along the length of young
roots. Feeding along roots produces symptoms similar to those caused by other
cortical feeding nematodes (disintegration of the cortex); those that feed at
root tips produce very different symptoms, including root-tip galling and
Xiphinemabacter is an
The pathogenicity of X. index is well-defined, but that of the
other species has not been studied, and several are known only from survey data
(Raski, 1988). Xiphinema index is a
stages feed at the root tip. The odontostyle penetrates deep into the
meristematic region of the root tip where secretions of the esophageal
glands result in cell hypertrophy and thickening. Kirkpatrick et al. (1965)
showed top and root reductions of grape by 65 and 38% respectively in grape pot
tests with X. index.
Xiphinema diversicaudatum also feeds at the root tip and causes
galling. In contrast, X. americanum sensu stricto feeds
behind the root tip and has not been associated with root tip galling.
Many species of Xiphinema have wide host ranges.
little critical biological and ecological work has been done on this important
genus - some species are difficult to maintain in greenhouse cultures, e.g. X.
americanum, others apparently have a very long life-cycle, e.g.,
X. diversicaudatum.. All stages
occur in the soil.
Eggs, from which first-stage juveniles
emerge, are deposited singly in water films around soil particles and are not
enclosed in an egg-mass. There are three or four juvenile stages and sexually
mature adults. Males are rare in most species and are apparently unnecessary for
Palomeres-Rius et al (2016) screened 124 plant-feeding nematode populations
for the presence of bacterial endosymbionts; that is, bacteria living within the
body of the nematode. Potential bacterial endosymbionts were only
detected in species of Xiphinema, particularly in species of the X.
americanum (sensu lato) group.
One group of 16S rRNA sequences was related to ‘Candidatus Xiphinematobacter’
and a second to the bacterial family Burkholderiaceae, which includes fungal
and plant endosymbionts.
The study suggests a highly specific symbiotic relationship and a high degree
of phylogenetic congruence and long-term evolutionary association between hosts
The bacterial endosymbionts are transmitted vertically to progeny. They
are observed to colonize the ovary wall of females of Xiphinema
species, they apparently invade developin oocytes and are seen in
uninucleate eggs in the uterus. They become associated with intestinal
epithelium during nemattode development and then with the overy in adult females
(Palomares-Rius, 2016; Vandekerckhove et al., 2002; Brown et al., 2015).
Endosymbionts may have effects that range from parasitism (harmful to the
host), through commensalism (neutral effects) to mutualism (beneficial to host
and enosymbiont). The effects of the realtionships between the
endosymbionts and Xiphinema spp. is unknown at this time)s. In
the latter case, both host and endosymbiont would benefit from the
realtionship.Although the nat
There are other examples of bacterial endosymbionts in nematodes. In
many cases their roles in the biology of the nematode are unknown
(Palomares-Rius et al, 2016), for example:
Associations between the insect-pathogenic bacteria
Photorhabdus and Xenorhabdus is necessary for completion of the life cycle
of Heterorhabditis and Stienernema (Ruby. 2008).
Detection of Wolbachia in the burrowing nematode
Radopholus similis and R. arabocoffeae (Haegeman, et al.
Verrucomicrobia has been detected in theXiphinema
americanum group (Vandekerckhove et al., 2002).
Bacteroidetes has been detected in populations
of Heterodera glycines and Globodera rostochiensis (Noel &
Species of Xiphinema
are primarily problematic in biennial and permanent crop situations. Except for
damage to emerging seedlings, they are seldom of major importance in annual
crops as they rarely attain high population levels when soil is frequently
tilled. However, in the case of species that are vectors of plant viruses, the
damage casused by the viruses may be independent of the abundnace of nematodes.
Feeding along roots produces symptoms similar to other cortical
feeders (distintegration of cortex); those that feed at root tips produce
very different symptoms, i.e., root stunting. There is also ample evidence
that the nematode injects substances into roots which causes root
swelling. Damage to roots causes foliage to become stunted.
Control of Xiphinema spp. on many
annual crops may not be of major importance since species of this nematode seldom
attain high population levels when soil is frequently tilled.