Plant biology Essay Example | Topics and Well Written Essays - 1250 words

Plant biology - Essay Example HGT is observed mainly between prokaryotes (single-celled organisms) like bacteria but also can be in Eukaryotes (protists, fungi, plants, animals, humans) to a lesser extent. HGT was first described by Ochiai et al. in 1959, in relation to transfer of antibiotic resistance between different bacterial species. Thereafter many scientists have studied and discussed about this phenomenon and now it is accepted that HGT is not a rare event and what is present in the biological world today is not a result of vertical gene transmission alone but also of HGT. Biological kingdom of earth can be categorized into three domains as bacteria, archea and eukaryotes. During vertical gene transmission, genetic information exchange between members of these domains is restricted to closely-related organisms with homologous DNA sequences. Therefore the ancestry of an organism could be traced back by analyzing their DNA and individuals with similar gene sequences can be grouped together to construct the evolutionary pathway. However, as HGT can transfer genes across wide phylogenetic distances, this picture become obscure. The phylogenetic tree of life (Figure 1) which present the relationship among different biological taxa is thus complicated by HGT as numerous interconnecting branches became obvious due to the presence of homologous DNA sequences in distantly-related organisms (Simonson et al.). Transfer of genes between different biological kingdoms, such as between eucaryotes and bacteria, or between bacteria and insects are some extreme cases of gene exchange between wide phylogenetic distances. An example is the presence of bacterial 'rol' genes from Agrobacterium species in tobacco (Nicotiniana) plants (Intrieri and Buiatti). Figure 1. Phylogenetic tree of life as proposed by Carl Woese In vertical gene transmission, genetic variability of a species mainly arises during meiosis by recombination of genes. According to Mendal’s law of inheritance, alleles of different genes assort independently during gamete formation and thereby produce novel gene combinations which in turn generate variability. Mutations, which are the random changes of gene sequence of a DNA strand, are considered as an error in the vertical gene transmission process. It can happen as a point mutation, chromosomal duplication, breaking and rearrangement of chromosomes and addition or deletion of chromosomes. When such mutant is passed down to the next generation, it is not a mutant anymore, but a variant or a novel type. The variants who survive according to Darwin’s theory of â€Å"survival of the fittest†, subsequently pass the novel characters to their offspring by sexual or asexual reproduction. HGT however, is not a mutation and is not involved in gamete formation or sexual reproduction. It moves transposable elements between distantly-related or unrelated genomes and thereby intervenes in the process of evolution by originating different cell types and cre ating variants in its own way. Two hypotheses are presented here as ‘Continual horizontal transfer hypothesis’ and ‘Early massive horizontal transfer hypothesis.’ (Jain, Rivera and Lake). According to the ‘Continual horizontal transfer hypothesis’, HGT is a continuous process during prokaryotic evolution whereas in the ‘Early massive horizontal transfer hypothesis’, a massive exchange of a few operational genes (genes involved in