Reproductive isolation hybridization

Hybridization has retained oaks a model taxon for species concepts that rely on ecological criteria rather than on reproductive isolation [65]. The high similarity in chemical constitution of different oak pollens, which is verified by a very similar biochemical fingerprint in our Raman experiments, can be interpreted as part of the very weak barriers to being pollinated by another (oak) species. 

Like other insects, lepidopterans bear cuticular hydrocarbons to protect them against desiccation. These hydrocarbons can be used as taxonomic markers (Table 7.6). In the Danaus genus, D. erippus and D. plexippus show reproductive isolation. Some taxonomists view them as separate species, but others consider D. erippus as a subspecies of D. plexippus. Hybridization experiments showing prezygotic and postzygotic isolation, as well as differences in CHC chromatograms, strongly support the hypothesis that the two species are separate (Hay-Roe et al., 2007). 

As indicated earlier H. subflexa has recently become the subject of numerous pest control studies due to the potential for population control of H. virescens by a sterile hybrid release program. However, these two species maintain reproductive isolation despite broadly overlapping ranges and intersecting reproductive periods. Hence, barriers to interspecific gene flow are most likely due to differences in their respective sex pheromone communication systems and, as indicated earlier, there are considerable differences in the pheromone gland contents of each. 

Species-specific pheromone trails would be a useful prezygotic reproductive isolating mechanism as they would decrease the chance of hybrid matings. Such specificity would also save the male the energy of trailing a nonreceptive heterospecific female and reduce the male s exposure to predators. It seems likely that all species of snakes using trails in a... 

Fisher et al. (1996b) were able to isolate two cDNAs from an Arabidopsis thaliana hypocotyl library that seemed to be those coding for BEIIa and BEIIb. The two cDNAs had diverged 5 and 3 ends, but the amino-acid sequences encoded by them were 90% identical. The two cDNAs hybridized to transcripts that showed similar expression patterns in the vegetative and reproductive tissues. 

The role of sexuality in the speciation process is unclear. C. purpurea was proved to be homothallic, which should (similarly to asexual reproduction) support separation of the population in clonal lineages. Small variability of C. gigantea and C. africana shows that this is not occurring. On the other hand, no isolates that might be hybrids of ecoraces have been found in C. purpurea so far, maybe due to effective separation of the populations, which rarely occur on the same locality. 

We now present two examples cultivated barley (Hordeum vulgare) s susceptible to mildew Erisyphe graminis var. hordei), but Hordeum bulbosum is resistant. Hybrids were desired and yet after a species cross the caryopses frequently had no endosperm and the embryo was dead. However, when the embryos were isolated and cultivated in culture media, reproductive hybrids were obtained. 

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