Long-term balancing selection for pathogen resistance maintains trans-species polymorphisms in a planktonic crustacean.

Balancing selection is an evolutionary process that maintains genetic polymorphisms at selected loci and strongly reduces the likelihood of allele fixation. When allelic polymorphisms that predate speciation events are maintained independently in the resulting lineages, a pattern of trans-species polymorphisms may occur. Trans-species polymorphisms have been identified for loci related to mating systems and the MHC, but they are generally rare. Trans-species polymorphisms in disease loci are believed to be a consequence of long-term host-parasite coevolution by balancing selection, the so-called Red Queen dynamics. Here we scan the genomes of three crustaceans with a divergence of over 15 million years and identify 11 genes containing identical-by-descent trans-species polymorphisms with the same polymorphisms in all three species. Four of these genes display molecular footprints of balancing selection and have a function related to immunity. Three of them are located in or close to loci involved in resistance to a virulent bacterial pathogen, Pasteuria, with which the Daphnia host is known to coevolve. This provides rare evidence of trans-species polymorphisms for loci known to be functionally relevant in interactions with a widespread and highly specific parasite. These findings support the theory that specific antagonistic coevolution is able to maintain genetic diversity over millions of years.

Exploring the mechanisms of host-specificity of a hyperparasitic bacterium (Pasteuria spp.) with potential to control tropical root-knot nematodes (Meloidogyne spp.): insights from Caenorhabditis elegans.

Plant-parasitic nematodes are important economic pests of a range of tropical crops. Strategies for managing these pests have relied on a range of approaches, including crop rotation, the utilization of genetic resistance, cultural techniques, and since the 1950's the use of nematicides. Although nematicides have been hugely successful in controlling nematodes, their toxicity to humans, domestic animals, beneficial organisms, and the environment has raised concerns regarding their use. Alternatives are therefore being sought. The Pasteuria group of bacteria that form endospores has generated much interest among companies wanting to develop microbial biocontrol products. A major challenge in developing these bacteria as biocontrol agents is their host-specificity; one population of the bacterium can attach to and infect one population of plant-parasitic nematode but not another of the same species. Here we will review the mechanism by which infection is initiated with the adhesion of endospores to the nematode cuticle. To understand the genetics of the molecular processes between Pasteuria endospores and the nematode cuticle, the review focuses on the nature of the bacterial adhesins and how they interact with the nematode cuticle receptors by exploiting new insights gained from studies of bacterial infections of Carnorhabditis elegans. A new Velcro-like multiple adhesin model is proposed in which the cuticle surface coat, which has an important role in endospore adhesion, is a complex extracellular matrix containing glycans originating in seam cells. The genes associated with these seam cells appear to have a dual role by retaining some characteristics of stem cells.

Natural occurrence of Pasteuria nishizawae in soybean areas / Ocorrência natural de Pasteuria nishizawae em áreas de soja

Soybean (Glycine max (L.) Merrill) is a widely grown crop of economic prominence. Brazil is considered as the second largest worldwide producer and exporter. Soybean cyst nematode, Heterodera glycines Ichinohe, in one of the most serious threats for this crop, considered as its most destructive parasite. The first report of this disease in Brazil was recorded during the 1991/1992 harvest. Control of nematodes is more challenging if compared with other plant diseases control. Thus, there is a growing demand to search for alternative control practices that will not harm the environment nor the human being. Therefore, the highly specific bacteria Pasteuria spp. Metchnikoff, represents an auspicious biological control agent against nematodes. The biological control of the soybean cyst nematode with the bacterium Pasteuria nishizawae Sayre has proven an excellent choice and has been studied by different scientists. The objective of this work was to determine the natural occurrence of Pasteuria nishizawae in Brazilian soils. The experiment was performed under greenhouse conditions at the Research unit from Syngenta® in Uberlandia-MG, Brazil, with soil samples originated from soybean planted areas from the Brazilian states of Bahia, Goiás, Mato Grosso and Paraná. Fractions of 150 cm3 of soil were withdrawn to be processed by the centrifugal flotation technique in sucrose solution. Aliquots of 1 mL from the obtained suspension were observed in Peters' chamber with the aid of an inverted light microscope, in order to verify the absence or presence of bacterial endospores attached to the cuticle of the nematodes extracted from the soil samples. The frequency of Pasteuria nishizawae incidence was of 100% within the samples analyzed.

A soja (Glycine max (L.) Merrill) é uma cultura de grande importância econômica. O Brasil é o segundo maior produtor e exportador mundial. Um dos mais sérios problemas fitossanitários desta cultura é o nematoide do cisto, Heterodera glycines Ichinohe, considerado o parasito mais destrutivo. A primeira ocorrência desta doença no Brasil foi relatada na safra 1991/1992. O controle de nematoides é mais difícil quando comparado com outras doenças. Torna-se cada vez mais importante a busca de controle alternativo, que não ofereça risco ao meio ambiente e nem ao aplicador. Sendo assim, a bactéria Pasteuria spp. Metchnikoff com alta especificidade, representa um promissor agente de controle biológico dos nematoides. O controle biológico do nematoide do cisto da soja pela bactéria Pasteuria nishizawae Sayre vem sendo estudado por diversos pesquisadores. O objetivo deste trabalho foi determinar a ocorrência natural de Pasteuria nishizawae em solos brasileiros. O ensaio foi conduzido em casa-de-vegetação na Unidade de Pesquisa da Syngenta em Uberlândia ­ MG, com amostras de solo provenientes de áreas de cultivo de soja dos estados brasileiros Bahia, Goiás, Mato Grosso e Paraná. Foram retiradas uma alíquota de 150 cm3 de solo e então a mesma foi processada pela técnica da flotação centrífuga em solução de sacarose. Da suspensão obtida foi observada uma alíquota de 1 mL em câmara de Peters, com o auxílio do microscópio invertido, verificando a presença ou ausência de endósporos da bactéria aderidos na cutícula dos nematoides extraídos. A frequência de ocorrência da Pasteuria nishizawae foi de 100% nas amostras analisadas.