Crosses prior to parthenogenesis explain the current genetic diversity of tropical plant-parasitic Meloidogyne species (Nematoda: Tylenchida)

Abstract

The tropical and subtropical parthenogenetic plant-parasitic nematodes Meloidogyne are polyphagous major agricultural pests. Implementing proper pest management approaches requires a good understanding of mechanisms, population structure, evolutionary patterns and species identification. A comparative analysis of the mitochondrial vs nuclear diversity was conducted on a selected set of Meloidogyne lines from various geographic origins. Mitochondrial co2-16S sequences and AFLP markers of total DNA were applied because of their ability to evidence discrete genetic variation between closely related isolates. Several distinct maternal lineages were present, now associated with different genetic backgrounds. Relative discordances were found when comparing mitochondrial and nuclear diversity patterns. These patterns are most likely related to crosses within one ancestral genetic pool, followed by the establishment of parthenogenesis. In this case, they mirror the genetic backgrounds of the original individuals. Another aspect could be that species emergence was recent or on process from this original genetic pool and that the relatively short time elapsed since then and before parthenogenesis settlement did not allow for lineage sorting. This could also be compatible with the hypothesis of hybrids between closely related species. This genetic pool would correspond to a species as defined by the species interbreeding concept, but also including the grey area of species boundaries. This complex process has implications on the way genotypic and phenotypic diversity should be addressed. The phenotype of parthenogenetic lines is at least for part determined by the ancestral amphimictic genetic background. A direct consequence is, therefore, in terms of risk management, the limited confidence one can have on the direct association of an agronomic threat to a simple typing or species delineation. Risk management strategies and tools must thus consider this complexity when designing quarantine implementation, resistance breeding programmes or molecular diagnostic.

Introduction

The tropical and subtropical plant nematodes Meloidogyne are extensively studied polyphagous major agricultural pests. Different types of variability, i.e. morphological, physiological, host races … are recognized between and even within species and discussions have risen about species delineation (Lamberti, 1979, Sasser, 1979, Taylor and Netscher, 1979, Netscher, 1983, Sturhan, 1983). This is especially true within the tropical group of species, which reproduce through mitotic obligatory parthenogenesis (Triantaphyllou, 1963, Triantaphyllou, 1966, Triantaphyllou, 1981, Triantaphyllou, 1985, Rammah and Hirschmann, 1988). More recently, the focus on differentiation, especially through the use of scanning electron microscopy or molecular techniques, led to a more accurate typing and to the description of novel species in addition to the five widespread and economically important species traditionally recognized, i.e. M. incognita, M. javanica, M. arenaria, M. hapla (Eisenback, 1985) and M. mayaguensis (Rammah and Hirschmann, 1988).

Total DNA analysis of representatives of these five species evidenced a strong structure associating, in genetically isolated clusters, different lines also displaying what could be considered intraspecific diversity (Fargette et al., 1996, Fargette et al., 2005, Semblat et al., 1998). Hugall et al. (1997) reported a low diversity in mitochondrial DNA, but Stanton et al. (1997) and Blok et al. (2002) also demonstrated examples of size plasticity in the mitochondrial DNA region of M. incognita and M. arenaria. Confronting these initial observations thus leads to the suggestion that mitochondrial diversity might be larger than initially suspected.

Among the M. incognita, M. javanica, M. arenaria group, some questionable associations have been observed between mitochondrial data and nuclear supported information such as ITS variation (Hugall et al., 1999) and, more generally, morphology or host ranges characteristics (Hugall et al., 1994). This raises the question of how the diversity is organized within this group and which processes led to such a diversity pattern. Involvement of pre- vs post-parthenogenesis events has been discussed (Triantaphyllou, 1985, Hugall et al., 1994, Hugall et al., 1999, Castagnone-Sereno, 2006). Van der Beek and Pijnacker (2008), who used a mitotic isolate as a control in his work, showed that the mutational rate is very low in mitotic lines. If the mutational process cannot thus explain the observed diversity, there is as yet no clear indication on the process actually involved.

The status of agronomic pest is a direct consequence of selected traits. Implementing proper pest management approaches requires a good understanding of mechanisms, population structure, evolutionary patterns and species identification. Associating a threat to a given genotype or genetic pool is therefore a key issue. Gaining knowledge on diversity patterns and species variation and delineation is thus essential to improve management and control of such pests.

In line with this question we report here a genomic analysis and a comparative analysis of mitochondrial DNA co2-16S sequences and AFLP markers of total DNA from a selected set of Meloidogyne lines of various geographic origins. We demonstrate here that the patterns (including some “relative discordances”) observed in population structure of parthenogenetic lines of Meloidogyne show the signature of crosses prior to the establishment of parthenogenesis. We also demonstrate that several distinct maternal lineages exist, now associated with different genetic backgrounds.