Soil nematode abundance and functional group composition at a global scale.

Soil organisms are a crucial part of the terrestrial biosphere. Despite their importance for ecosystem functioning, few quantitative, spatially explicit models of the active belowground community currently exist. In particular, nematodes are the most abundant animals on Earth, filling all trophic levels in the soil food web. Here we use 6,759 georeferenced samples to generate a mechanistic understanding of the patterns of the global abundance of nematodes in the soil and the composition of their functional groups. The resulting maps show that 4.4 ± 0.64 × 1020 nematodes (with a total biomass of approximately 0.3 gigatonnes) inhabit surface soils across the world, with higher abundances in sub-Arctic regions (38% of total) than in temperate (24%) or tropical (21%) regions. Regional variations in these global trends also provide insights into local patterns of soil fertility and functioning. These high-resolution models provide the first steps towards representing soil ecological processes in global biogeochemical models and will enable the prediction of elemental cycling under current and future climate scenarios.

Morphological and molecular characterization of Hoplolaimus pararobustus (Schuurmans Stekhoven and Teunissen, 1938) Sher 1963 with its first report on Zea mays roots in Namibia.

In the summer of 2018, specimens of a Hoplolaimus population were extracted from a maize root sample collected near Stampriet, Namibia. This population was identified as Hoplolaimus pararobustus and is described and illustrated based on its morphological, morphometric, and molecular characteristics. To our knowledge, this is the first report H. pararobustus from maize roots. Females of the population had a mean body and stylet length of 1,100 µm and 36 µm, respectively. Esophagus with three nuclei in three pharyngeal glands. Lateral field reduced, ranging from a very faint line to just breaks in striae. The males were shorter than the females with a mean body length of 925 µm and the stylet slightly shorter, with a mean length of 34 µm. Phylogenetic analyses using partial sequences of 18 S and the expansion fragment D2-D3 of 28 S rDNA genes showed the close relation of this species and H. columbus. This Namibian population of H. pararobustus is the first Hoplolaimus species from Africa to be molecularly characterized.

Nematodes and the effect of seasonality in grassland habitats of South Africa.

Nematodes in South Africa have mainly been studied for their diversity and agricultural importance. However, the ecological status of nematodes and the effect of seasonal variation in local grasslands remain unknown. For this reason, a nematode study was conducted in the Telperion Nature Reserve and represented the first ecological study in a natural grassland area in South Africa. In total, 104 soil samples were collected during four consecutive seasons from 2015 until 2016 in three habitats, viz. (i) open grassland, (ii) shrubland with rocky outcrops, and (iii) riparian zone. From these the nematode community structure and soil ecosystem status were studied. In total, 93 genera from 50 families were recorded with herbivores and bacterivores being the most abundant trophic groups in all three habitats. Linear mixed models revealed that season had an overwhelmingly dominant impact on the condition, food web status, and functioning of the soil ecosystems with pairwise comparisons indicating that significantly higher values were recorded during winter. Interestingly, this seasonal shift can largely be attributed to fluctuations in the populations of only a few nematode groups (namely Aporcelaimellus, Dorylaimidae, Iotonchus, and Mononchus) with high colonizer-persister values. Although the reason for the higher abundance of specific nematode groups recorded during the winter is not explicitly clear, it is possibly linked to reduced competition from other soil fauna. This study clearly shows that further investigations are required to better understand the dynamics of grassland ecosystems.

Molecular and morphological characters of Helicotylenchus Steiner, 1945 species from Iran with a note on the identity of H. pseudorobustus (Steiner, 1914) Golden, 1956.

Helicotylenchus species were recovered from natural areas of three provinces (Mazandaran, Kerman and Guilan) in Iran, with morphological and molecular characteristics suggesting that they were Helicotylenchus minzi and Helicotylenchus pseudorobustus. A phylogenetic analysis of the two Helicotylenchus species using 28S rDNA placed H. pseudrobustus and H. minzi in a clade with 1.00 posterior probability. Results suggest that the Helicotylenchus population identified using molecular characteristics as H. pseudorobustus is a paraphyletic group, with hierarchical clustering analysis indicating a close relationship between the Iranian and the Chinese populations of H. pseudorobustus. In conclusion, further molecular studies within the genus Helicotylenchus are needed, especially for morphologically similar populations that have been recognized as separate species.

Molecular characterization of Helicotylenchus multicinctus and H. dihystera (Tylenchida: Hoplolaimidae) from Theobroma cacao in Nigeria.

The genus Helicotylenchus contains cosmopolitan, ubiquitous plant-parasitic nematodes with some species capable of causing significant economic damage to agricultural crops. Accurate species identification in this genus is essential in recognizing the damaging species and establishing effective management options. In a study on cocoa plantations in Nigeria, two species of spiral nematodes were found in high numbers from soil samples obtained from a six-decade old cocoa plantation at the Cocoa Research Institute of Nigeria. An integrated approach involving a combination of morphology, morphometrics, and molecular tools was used to identify the nematode species. Morphological data indicate the presence of both H. multicinctus and H. dihystera. There is congruence in the morphological and molecular data obtained for H. multicinctus. However, phylogenetic analysis of the 28S rDNA expansion segment revealed a high variability in the sequences of the Nigerian population of H. dihystera, suggesting the need for a careful appraisal and more comparative studies.

Additive interaction between a root-knot nematode Meloidogyne javanica and a root-feeding flea beetle Longitarsus bethae on their host Lantana camara.

BACKGROUND: Changes in plants induced by one species have indirect effects on interactions with other species, thus shaping their abundances. The root-feeding beetle Longitarsus bethae released as a biological control agent for Lantana camara has established at a few sites in South Africa. Lantana camara plants infected with the root-knot nematode Meloidogyne javanica often exhibit an increase in L. bethae abundance, suggesting that nematode-infected plants could be enhancing the development of the beetle. This study investigated the interaction between L. bethae and M. javanica. RESULTS: The study showed that galling by M. javanica occurred at the highest inoculation of 300 eggs of L. bethae per plant. Longitarsus bethae performed over twofold better on M. javanica-infected L. camara roots compared with healthy roots, and 275 adult L. bethae progeny, with slightly larger body size, emerged from M. javanica-infected compared with 167 adults that emerged from healthy plants. Fresh gall weight from treatments where both L. bethae and M. javanica were combined was 36% higher than that from M. javanica only, suggesting that the combination of both species induces more galling than the nematode does alone. The total biomass of plants with M. javanica only, L. bethae only, and the two species combined was reduced by 48%, 24% and 50%, respectively. CONCLUSION: Meloidogyne javanica improves the performance of L. bethae, and combination of the two species has an additive negative effect on L. camara. The interaction between M. javanica and L. bethae could enhance the biological control of L. camara. © 2019 Society of Chemical Industry.

A global database of soil nematode abundance and functional group composition.

As the most abundant animals on earth, nematodes are a dominant component of the soil community. They play critical roles in regulating biogeochemical cycles and vegetation dynamics within and across landscapes and are an indicator of soil biological activity. Here, we present a comprehensive global dataset of soil nematode abundance and functional group composition. This dataset includes 6,825 georeferenced soil samples from all continents and biomes. For geospatial mapping purposes these samples are aggregated into 1,933 unique 1-km pixels, each of which is linked to 73 global environmental covariate data layers. Altogether, this dataset can help to gain insight into the spatial distribution patterns of soil nematode abundance and community composition, and the environmental drivers shaping these patterns.

Morphological and morphometrical identification of Meloidogyne populations from various crop production areas in South Africa with emphasis on M. enterolobii.

Accurate identification of Meloidogyne spp. is crucial and the first step to apply suitable management strategies to combat these nematode pests. Perineal-pattern morphology of female specimens is one of the most common characteristics used for identification. However, for some species various morphological characteristics are similar which makes it challenging to correctly identify species. In this study different morphological and morphometrical characteristics were used to identify 37 populations of Meloidogyne obtained during 2015 and 2016 from various crop production areas situated across different geographical regions in South Africa. A comprehensive study of females, males and second-stage juveniles (J2) of the 37 Meloidogyne populations isolated was conducted, revealing the presence of Meloidogyne enterolobii, M. hapla, M. incognita and M. javanica. Although three perineal-pattern characteristics proved to be useful in discriminating particularly between M. enterolobii and M. incognita females, most of the morphometric characters used to identify female, male and J2 individuals overlapped among the different species. Substantial intraspecies variation was also evident among different populations. The use of classical identification approaches alone could therefore not clearly distinguish among the 37 Meloidogyne populations studied. Therefore, the use of molecular techniques in combination with morphological and morphometrical analyses is suggested to be more accurate and reliable in discriminating between Meloidogyne spp.

Description of two new species, Aphanolaimus strilliae n. sp. (Nematoda:br />Aphanolaimidae) and Makatinus africanus n. sp. (Nematoda: Aporcelaimidae), and SEM observations on three known species from freshwater sources in the Telperion Nature Reserve (Mpumalanga, South Africa).

Aphanolaimus strilliae n. sp. and Makatinus africanus n. sp. are described from freshwater sources in the Telperion Nature Reserve, Mpumalanga, South Africa. Aphanolaimus strilliae n. sp. is characterised by a body length of 1240-1613 µm, more than 800 body annules, lateral field originating between the first and second lateral body pore at the 34th-46th annule, first lateral body pore located at the 25th-35th annule, vagina V-shaped and bent anteriorly, 142-195 µm long uterus and 165-207 µm long tail with spinneret offset. This species is ovoviviparous and no males were found. Makatinus africanus n. sp. is characterised by a large, thick body (3228-4128 µm long, a = 30-39 wide), slightly set off lip region with amalgamated lips; 31-34 µm long odontostyle; long tongue-shaped cardia, and stout, short tail (30-43 µm long) with a small peg / digitate extension and male absent. Populations of three known species, Chronogaster africana, Eutobrilus annetteae and Neotobrilus ampiei, from fresh water at the Telperion Nature Reserve are described and scanning electron microscope graphs of these species published for the first time.

Aphelenchoides spp. (Nematoda: Aphelenchida) and Panagrolaimus leperisini (Nematoda: Rhabditida) found associated with grass seeds in the Telperion Nature Reserve, South Africa.

As part of the South African Plant-Parasitic Nematode Survey (SAPPNS), an extensive nematode diversity survey was conducted in the Telperion Nature Reserve, Mpumalanga, South Africa. During this survey, samples of grass seeds were collected. Some of these seeds yielded specimens of the genera Aphelenchoides, Aphelenchus and Panagrolaimus. Identified species include Aphelenchoides africanus, A. besseyi, A. lichenicola, A. rutgersi, A. spicomucronatus and Panagrolaimus leperisini. Four of these species are new records for South Africa and the South African populations of these species are described in this paper.

Morphological and molecular characterization of three Helicotylenchus species from Iran.

Three Helicotylenchus species, namely Helicotylenchus digonicus, Helicotylenchus pseudorobustus and Helicotylenchus vulgaris were identified during a nematode survey conducted in Iran. The morphological and molecular characteristics of the species were defined. In addition, phylogenetic analysis of five Iranian Helicotylenchus populations based on 28S rDNA were done and showed the unresolved position of molecularly identified H. pseudorobustus and H. digonicus. However, populations of H. vulgaris and H. digonicus were placed close to each other. Phylogenetic analysis confirmed that molecularly characterized Helicotylenchus species formed a monophyletic clade. A detailed revision of the species is, however, needed. Illustrations and measurements for all species are provided.

Genetic diversity and phylogeny of South African Meloidogyne populations using genotyping by sequencing.

Meloidogyne species cause great crop losses worldwide. Although genetic host plant resistance is an effective control strategy to minimize damage caused by Meloidogyne, some resistant genes are ineffective against virulent species such as Meloidogyne enterolobii. Detailed knowledge about the genetic composition of Meloidogyne species is thus essential. This study focused on genotyping-by-sequencing (GBS) and Pool-Seq to elucidate the genetic relation between South African M. enterolobii, M. incognita and M. javanica populations. Hence, 653 common single nucleotide polymorphisms (SNPs) were identified and used to compare these species at genetic level. Allele frequencies of 34 SNPs consistently differed between the three Meloidogyne species studied. Principal component and phylogenetic analyses grouped the M. enterolobii populations in one clade, showing a distant relation to the M. javanica populations. These two species also shared genetic links with the M. incognita populations studied. GBS has been used successfully in this study to identify SNPs that discriminated among the three Meloidogyne species investigated. Alleles, only occurring in the genome of M. enterolobii and located in genes involved in virulence in other animal species (e.g. a serine/threonine phosphatase and zinc finger) have also been identified, accentuating the value of GBS in future studies of this nature.