Polyploids are found to play a major role in genetic, evolutionary and systematic studies. The patterns of multiple origins and development of polyplidy in plants have been of considerable research interest. Recent studies are focusing on the evolutionary consequence of gene duplicaton in polyploids, gene expression patterns, and comparisons on variations in gene regulation in natural and artificial polyploids.
Polyploidy is the presence of multiple sets of genomes in the same individual. This process has been found to be one of the major causes of speciation and hence evolution. Polyploidy is one of the major evolutionary forces of angiosperm development.
Among plant species, polyploids are found to occur mainly in ferns (95%) and angiosperms (40-50%). There is only one conifer species (Sequoia) in which polyploidy is found. It occurs as a hexaploid or autoallopolyploid.
Polyploidy may be either autopolyploidy or allopolyploidy. The polyploidy genome sets undergo drastic changes in the structure and function of genome through epigenetic changes and genetic changes. Genetic changes occur through the processes of translocation, deletion, transposition, mutations and loss of DNA sequence. Epigenetic modifications occur via regulatory pathways mediated by RNA (RNA interference) and Chromatin remodeling.
Polyploids has been found to be of single or multiple origins. Species of single origin include wheat, peanut, Arabdopsis etc. Those with single and multiple origins differ in processes of gene silencing and gene expression.
The effects of polyploidy are evident in plant species. Some of them are discussed here.
Polyploids in their course of evolution have been shown to become diploids both in cytogenetic and genetic makeup. This finding has given rise to the term paleopolyploids which indicate the diploid species which were polyploids in the past. Examples include rice, maize, Arabidopsis etc.
The earlier researches in this field have shown that about 2-4% of the speciation events in angiosperms were due to polyploidy. The phenomenon was also found in 7% of the speciation events in case of ferns. Sympatric speciation in plants was thought to be mainly due to polyploidy.
With the new sequencing methods and comparison algorithms, polyploidy was found to be involved in the speciation of flowering plants and eukaryotes too. Presence of gene redundancy in these organisms was attributed to either polyploidy or whole genome duplication. Polyploidy has been found to be important in the evolution of amphibians also.
The evidences point out to the lineages undergoing repeated polyploidization followed by diploidization. The process of polyploidization is thought to be followed by other processes such as genomic rearrangement or gene silencing which will facilitate diploidization.
c. Conservation of species
Although the recurrence of polyploidization was evident, the frequency and importance of such events need to be studies in detail. The recurrence was found to counterbalance the local extinction of initial autopolyploid species.
d. Difference in gene composition
Soybean species and its relatives are complex paleopolyploids which had undergone two polyploidization events in their evolutionary history. The homeologous species of soybeans were found to differ in the transposon insertions and R-gene composition. Further evidences were found for polyploidy to start divergent evolution within the species.
e. Variations in gene expression
Gene expression pattern in gene duplication have shown that polyploidy genomes respond in similar manner to environmental stimuli. Hence, polyploidy and whole genome duplication has also been proposed to be the result of external factors.
f. Neofunctionalization and subfunctionalization
The major results of polyploidy include neofuncitonalization or subfunctionalization of genes. Neofunctionalization is the acquiring of new functions for the existing genes and subfunctionalization is the process of limiting the functions of the gene.
g. Epigenetic changes
Chromosomal rearrangements and DNA methylation pattern changes are frequent in polyploids and this has resulted in variations in the process of gene expression and regulation. Such changes in Brassica have resulted in considerable morphological variations.
Applications of Polyploidy
1. Study of adaptive evolution among polyploids reveal significant information on the evolutionary history of plants and hence can lead to better conservation methodologies.
2. Crop domestication is perhaps the most significant application of polyploidy since polyploids are found to be high in vegetative content. The only drawback is that further propagation needs clonal means.
3. Studies on polyploidy also can reveal information on how the plant genomes manage to succeed the effects of genome obesity (increase in genome dosage).
The effect and process of polyploidy need to be evaluated more. Future prospects and further applications of polyploidy are increasingly being discovered. The phenomenon once reserved for plants, has now been found to occur in animals alike.
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