Abstract
Taxonomically restricted genes (TRGs), i.e., genes that are restricted to a limited subset of phylogenetically related organisms, may be important in adaptation. In parasitic organisms, TRG-encoded proteins are possible determinants of the specificity of host-parasite interactions. In the root-knot nematode (RKN) Meloidogyne incognita, the map-1 gene family encodes expansin-like proteins that are secreted into plant tissues during parasitism, thought to act as effectors to promote successful root infection. MAP-1 proteins exhibit a modular architecture, with variable number and arrangement of 58 and 13-aa domains in their central part. Here, we address the evolutionary origins of this gene family using a combination of bioinformatics and molecular biology approaches. Map-1 genes were solely identified in one single member of the phylum Nematoda, i.e., the genus Meloidogyne, and not detected in any other nematode, thus indicating that the map-1 gene family is indeed a TRG family. A phylogenetic analysis of the distribution of map-1 genes in RKNs further showed that these genes are specifically present in species that reproduce by mitotic parthenogenesis, with the exception of M. floridensis, and could not be detected in RKNs reproducing by either meiotic parthenogenesis or amphimixis. These results highlight the divergence between mitotic and meiotic RKN species as a critical transition in the evolutionary history of these parasites. Analysis of the sequence conservation and organization of repeated domains in map-1 genes suggests that gene duplication(s) together with domain loss/duplication have contributed to the evolution of the map-1 family, and that some strong selection mechanism may be acting upon these genes to maintain their functional role(s) in the specificity of the plant-RKN interactions.
Highlights
The phylum Nematoda comprises over 25,000 described species, many of which are parasites of animals or plants [1]
The map-1 gene was originally identified in M. incognita, and a preliminary search among seven additional nematode taxa identified homologs in two closely related species only, i.e., M. arenaria and M. javanica [8]
We made use of greatly expanded sequence data sets from nematode genomes, in combination with PCR experiments, to look for homologs, in the Nematoda phylum, of the map1 gene originally identified in the root-knot nematode (RKN) species M. incognita [8]
Summary
The phylum Nematoda comprises over 25,000 described species, many of which are parasites of animals or plants [1]. Root-knot nematodes (RKNs), Meloidogyne spp., are extremely polyphagous species able to infect the roots of almost all cultivated plants, being responsible for estimated losses of more than 80 billions Euros/year [2]. These nematodes are obligatory, sedentary endoparasites which have evolved an intimate interaction with their hosts, by inducing the redifferentiation of root cells into permanent feeding sites [3]. It is generally admitted that sexual reproduction is the ancestral state, and that parthenogenetic species evolved from amphimictic ancestors [4,5], a still running debate exists on whether the establishment of parthenogenesis in these nematodes is of ancient or recent origin [6,7]. The most agriculturally widespread and damaging species are by far the three closely related, mitotic parthenogens M. incognita, M. javanica and M. arenaria
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
