Host races

There are many possible causes of chemical and nutrient variability in tree tissues (27,28) which result in a wide range of spatial arrays of suitable and unsuitable food for insects (29). Although large-scale spatial variation may influence insect host race formation and have interesting consequences for insect biogeography and host race formation (30), the scale of variation with which the individual insect deals most often is more local, on the individual tree or tissue basis. 

Zeiraphera diniana (Lepidoptera Tortricidae, the larch budmoth) polymorphism, host races, or sibling species. Heredity 75 416 124. 

In all the chemolocation assays with pea crabs, the attraction of crabs from each host race to their respective host was substantially less than 100%, and some crabs chose incorrect hosts, suggesting that there may still be at least the potential for gene flow among populations. These experiments were conducted with adult crabs, and host location experiments with larval or post-larval forms that are normally responsible for initial host selection might be particularly informative. Nevertheless, this series of studies suggests that within-species differences in chemically mediated host location may lead to population subdivision and reproductive isolation among marine species. 

Stevens, P.M., Specificity of host-recognition of individuals from different host races of symbiotic pea crabs (Decapoda Pinnotheridae), J. Exp. Mar. Biol. Ecol., 143, 193, 1990. 

Baltensweiler W. and Priesner E. (1988) A study of pheromone polymorphism in Zeiraphera diniana Gn. (Lep., Tortricidae) 3. Specificity of attraction to synthetic pheromone sources by different male response types from two host races. J. Appl. Entomol. 106, 217-231. 

Guerin P. M., Baltensweiler W., Arn H. and Buser H.-R. (1984) Host race pheromone polymorphism in the larch budmoth. Experientia 40, 892-894. 

Fanelli, D., Henshaw, M., Cervo, R., Turillazzi, S., Queller, D.C. and Strassmann, J.E. (2005). The social parasite wasp Polistes atrimandibularis does not form host races. J. Evol. Biol., 18, 1362-1367. 

A study of conidia of English C. purpurea collections revealed the similarity in conidial shape among the samples from certain host groups suggesting the existence of C. purpurea host races (Loveless, 1971). 

However, the question of host races and specificity of C. purpurea continues. Loveless (1971) found that the host-specific differences in conidial size and shape in English collections were retained even in laboratory cultures and corresponded to some extent to the Stager s groups. Following numerous crossinoculation experiments, Tanda allocated Japanese isolates of C. purpurea from different pooid and arundinoid hosts to four varieties C. purpurea van purpurea, van alopecuri, van phalaridis and van sasae (summarized in Tanda, 1979a, b). The latter variety occurs on a bambusoid host and could be probably species different from C. purpurea. 

Plants are continually exposed to a vast array of potential phytopathogenic fungi nevertheless, plants resist to most of them by blocking fungal development soon after penetration. Resistance against pathogens can be distinguished in resistance at the species level (non-host resistance) and resistance at the cultivar level (race-cultivar resistance). Plants lack a circulatory system and antibodies and have evolved a defense mechanism that is distinct from the vertebrate immune... 

The evolutionary history of symbiotic nitrogen fixers is therefore a tale of coevolution, which occurred in the shadow of their hosts, chasing their growing roots, and striving for adaptation. It is an example of how bacterial genetics has managed to keep pace with the creative power of eukaryotic sexual recombination. Mobile replicons, insertion elements, and symbiotic islands prone to move have helped rhizobia to succeed in their pursuit. The race, naturally, is not over and, looking at it from a distance, what we have. seen, compared to what we have yet to see, is probably just a cloud of dust. 

It is a bacterial pathogen that causes common wilt. There are live races of Ralstonia solanacearum with different hosts and geographic distributions. Race 3 is found worldwide except in the United States and Canada. 

In April 1972 women were allowed to participate in the Boston Marathon. West Germany hosted the first international marathon for women, in Waldniel, on 22 September 1974. In 1979, Joan Benoit from the USA won the first Olympic marathon race for women in Los Angeles in 1984 in 2 h 24 min 52 s. The current world s best time is held by Paula Radclijfe, 2 h 13 min 0 s achieved in the London Marathon. 

West Africans resident in Britain had average y-globulin levels for those in Britain less than two years of 2.2 g/100 ml as compared with 2.0 g/100 ml for those resident two to four years and 1.6 g/100 ml for those resident five to eight years (S3). Specific host factors in which the immune mechanism are implicated was believed to account for the higher levels of y-globulin in American Negroes and Puerto Ricans than in white races (S9). 

Highly resistant wheat varieties exhibit a typical hypersensitive response when infected with an avirulent race of the stem rust fungus. Host cells which are penetrated by a fungal haustorium undergo rapid necrotization, thus depriving the biotrophic parasite of its nutritional basis. 

When highly resistant wheat varieties are inoculated with an avirulent race of the stem rust fungus, fungal growth is arrested by the hypersensitive death of the first penetrated host cells (30,31.) Even in very densely inoculated leaves, the reaction of less than one percent of the host cells is sufficient to stop further development of the parasite. This small percentage may be the reason, why no increased content of biochemically determined lignin was measured in infected hypersensitive wheat leaves (60,61). 

The biochemical similarities of leaf and stem rust elicitors, the missing gene specificity of both elicitors, the fact that an equally active elicitor was isolated from germ tube cell walls of oat crown rust (60), together with the described reactions of different non-hosts to the stem rust elicitor led us to speculate that the isolated elicitors may play a role in the induction of general mechanisms of non-host resistance (130). Although they may be involved in the elicitation of race-cultivar specific resistance as well, race-cultivar specificity in the wheat-stem rust system clearly cannot be explained on the basis of the specificity of the isolated elicitors. One possible explanation would be the occurrence of race-cultivar specific suppressors of the resistance reaction (124,131,132). 

Figure 1. Hypothetical scheme of events leading to race-cultivar specific resistance or susceptibility in the rust system. If the substrate specificity of the fungal cell wall degrading enzymes (e.g., pectinases) is suitable for degradation of a specific host cell wall component (e.g., partly esterified pectin), endogenous suppressors will be produced which prevent the elicitor induced lignification response, thus leading to susceptibility.

The establishment of plant integrated defenses involves the preferential evolutionary retention and production of those SCs exerting synergistic toxic effects and is possible only if a diversification of secondary metabolism in a given plant has previously occurred. This preliminary diversification of secondary metabolism could be mediated via the classical reciprocal co-evolutionary interactions between a host plant and its major pests, as predicted by the chemical arms race model (Beren-baum and Zangerl, 1996). The PICD hypothesis is consequently not an exclusive evolutionary hypothesis because it is compatible with and dependent on other evolutionary processes. The contribution of the PICD hypothesis is to provide both a functional explanation for the diversity of SCs within plants (Romeo et al, 1996) and a reconciliation between different evolutionary models. 

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