Abstract
The fundamental molecular basis for phenotypic and genetic similarities among many described cases of plant hybrid necrosis has not been fully described. Plants can be good models for studying the basis of such gene flow barriers which occur between species. Many studies in prezygotic barriers like stigma recognition of pollen, environmental adaptation differences and pollinator preferences which can reduce the chances of species mating success have been done. Also studied are post zygotic barriers in gene flow like lack of ecosystem adaptation of hybrids which may include failure of pollinators from being attracted to floral parts due to developmental changes and gene or chromosome incompatibility resulting in genetic isolation. Polyploidy has also been recognized as an isolating force although it might not be the only post zygotic genome isolating force; other forces may also contribute hindrances in the gene flow after zygote formation. Here, papers which have tended to pinpoint the increasing evidence of epigenetic polymorphisms as causes of genomic conflicts which cause barriers to the gene flow resulting in hybrid necrosis in plants were reviewed. Explorations into the mechanisms which unlock hybrid necrosis can help plant breeders to avoid genetic and epigenetic incompatibilities during crop improvement programs. Key words: Hybrid necrosis, heterosis, epigenetic, hybrid sterility, RNAi.
Highlights
Little attention has been given to studies on postzygotic barriers in hybrid formation in plants especially hybrid necrosis which is caused by genetic incompatibility (Kirsten and Detlef, 2007a)
Evidence from recent studies on A. thaliana x A. lyrata hybrids showed that genetic self incompatibility (SI) breakdown was reversible with no apparent changes in the underlying DNA sequence it was concluded that these modifications were mainly epigenetic in their nature (Nasrallah et al, 2007)
Is exciting to imagine that the temperature sensitive RNA interference (RNAi) pathway could be involved in the type II necrosis, while other epigenetic mechanisms like cytosine DNA methylation be involved in type I hybrid necrosis
Summary
Little attention has been given to studies on postzygotic barriers in hybrid formation in plants especially hybrid necrosis which is caused by genetic incompatibility (Kirsten and Detlef, 2007a). It can be defined as a hybrid formation reproductive barrier that prevents proper exchange of genomic information between well established species or between varieties of one and the same species. During hybrid formation, necrosis affects the F1 generation but it can be observed in some F2 generation plants This phenomenon seems to clearly challenge the most accepted definition of “species” which is referred to as individuals which can mate to produce viable offspring (Weigel and Dangl, 2007) this reproductive barriers which prevent gene flow might at the same time set the stage for plant speciation (Gross, 2007; Sweigart et al, 2006)
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