Cultivated peanut (Arachis hypogaea) is an important crop, widely grown in tropical and subtropical regions of the world. It is highly susceptible to several biotic and abiotic stresses to which wild species are resistant. As a first step towards the introgression of these resistance genes into cultivated peanut, a linkage map based on microsatellite markers was constructed, using an F(2) population obtained from a cross between two diploid wild species with AA genome (A. duranensis and A. stenosperma). A total of 271 new microsatellite markers were developed in the present study from SSR-enriched genomic libraries, expressed sequence tags (ESTs), and by "data-mining" sequences available in GenBank. Of these, 66 were polymorphic for cultivated peanut. The 271 new markers plus another 162 published for peanut were screened against both progenitors and 204 of these (47.1%) were polymorphic, with 170 codominant and 34 dominant markers. The 80 codominant markers segregating 1:2:1 (P<0.05) were initially used to establish the linkage groups. Distorted and dominant markers were subsequently included in the map. The resulting linkage map consists of 11 linkage groups covering 1,230.89 cM of total map distance, with an average distance of 7.24 cM between markers. This is the first microsatellite-based map published for Arachis, and the first map based on sequences that are all currently publicly available. Because most markers used were derived from ESTs and genomic libraries made using methylation-sensitive restriction enzymes, about one-third of the mapped markers are genic. Linkage group ordering is being validated in other mapping populations, with the aim of constructing a transferable reference map for Arachis.

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Habenaria is a large genus of terrestrial orchids distributed throughout the tropical and subtropical regions of the world. The integrity and monophyly of this genus have been under discussion for many years, and at one time or another, several genera have been either included in a broadly defined Habenaria or segregated from it. In this study, the phylogenetic relationships of the Neotropical members of the genus and selected groups of African Habenaria were investigated using DNA sequences from the nuclear internal transcribed spacer (ITS) region and the plastid matK gene sampled from 151 taxa of Habenaria from the Neotropics (ca. 51% of the total) as well as 20 species of Habenaria and Bonatea from the Old World. Bayesian and parsimony trees were congruent with each other, and in all analyses, the Neotropical species formed a highly supported group. African species of Habenaria in sections Dolichostachyae, Podandria, Diphyllae, Ceratopetalae and Bilabrellae, and the Neotropical clade formed a highly supported "core Habenaria clade", which includes the type species of the genus from the New World. The topology of the trees indicates an African origin for the Neotropical clade and the low sequence divergence among the Neotropical species suggests a recent radiation of the genus in the New World. Species of Bonatea and Habenaria sections Chlorinae and Multipartitae formed a well-supported clade that was sister to the "core Habenaria clade". The Neotropical clade consists of at least 21 well-supported subgroups, but all Neotropical sections of the current sectional classification are paraphyletic or polyphyletic and will need extensive revision and recircumscription. Most of the Neotropical subgroups formed morphologically uniform assemblage of species, but some cases of morphological divergence within subgroups and convergence between subgroups indicated that morphology alone can be misleading for inferring relationships within the genus. The genera Bertauxia, Kusibabella and Habenella, segregated from New World Habenaria, are not monophyletic and a revision of the sectional classification rather than a generic division seems most appropriate. Our results do not support an extensive generic fragmentation of Habenaria as previously suggested and will provide a framework for revising the infrageneric classification and investigating the patterns of morphological evolution and geographical distribution of the genus in the New World.

The reproductive behaviour of the root-knot nematode Meloidogyne arenaria race 1 was compared on two wild species of Arachis ( A. duranensis and A. stenosperma ) and cultivated peanut ( A. hypogaea cv. IAC-Tatu-ST). The three species were considered moderately susceptible, resistant, and susceptible, respectively. Penetration and development of the root-knot nematode in the resistant species was reduced in comparison with that occurring in susceptible plants. Several cell features, including dark blue cytoplasm and altered organelle structure were observed in the central cylinder of A. stenosperma , indicating a hypersensitive-like response (HR) of infested host cells. Neither giant cells, nor nematodes developed beyond the second stage, were found on A. stenosperma. Arachis duranensis showed a delay in the development of nematodes in the roots compared to A. hypogaea. The two wild peanut species were chosen to be the contrasting parents of a segregating population for mapping and further investigation of resistance genes.