Polyploidy describes a biological condition where an organism possesses more than two complete sets of chromosomes in its cells, rather than the usual two sets found in diploid organisms. Allopolyploidy is a specific type of polyploidy where these extra chromosome sets come from two or more different parent species, resulting in a hybrid organism with a combined genome. While most eukaryotes are diploid, polyploidy is particularly common in the plant kingdom, playing a notable role in their diversity.
Formation of Allopolyploids
The formation of an allopolyploid involves a two-step process, beginning with the hybridization of two distinct parent species. This initial cross results in a hybrid offspring that contains one set of chromosomes from each parent. For example, if one parent species has a chromosome set ‘A’ and the other ‘B’, the hybrid would have an ‘AB’ chromosome complement.
These initial hybrids are often sterile or have reduced fertility because chromosomes from the two different species are not sufficiently similar to pair properly during meiosis, the cell division process that produces gametes. Without proper pairing, the gametes produced are non-viable or contain an unbalanced set of chromosomes. This sterility acts as a reproductive barrier, preventing the hybrid from interbreeding with either parent species.
The second step is the doubling of the chromosome set in the hybrid offspring. This doubling can occur spontaneously, for instance, through an error in cell division where chromosomes fail to separate. When the chromosome number doubles, the ‘AB’ hybrid becomes an ‘AABB’ individual, meaning it now has two copies of each chromosome set from both original parent species.
Chromosome doubling provides each chromosome with a homologous partner from its own original species. With homologous pairs available, proper chromosome pairing and segregation can occur during meiosis, leading to the production of fertile gametes. This restoration of fertility allows the allopolyploid to reproduce, either by self-fertilization or by mating with other individuals, thereby establishing a new, stable lineage.
Evolutionary Impact of Allopolyploidy
Allopolyploidy is a significant force in evolution, driving biodiversity, particularly within the plant kingdom. It can lead to rapid speciation in a relatively short evolutionary timeframe. This occurs because the allopolyploid often cannot successfully interbreed with its parent species due to differences in chromosome numbers, establishing reproductive isolation.
The combination of two genomes introduces novel genetic combinations and traits. This genetic redundancy, where genes are duplicated, can provide flexibility for evolutionary innovation. For instance, one copy of a duplicated gene might retain its original function, while the other copy can evolve new functions or expression patterns, leading to increased vigor or adaptation to new environmental conditions.
Allopolyploidy allows species to colonize new ecological niches. The new genetic makeup can provide a selective advantage, enabling allopolyploid species to thrive in environments unsuitable for their diploid ancestors. This ability to adapt to diverse habitats has contributed to the widespread occurrence of allopolyploids in nature, with some estimates suggesting that 30-80% of living plant species are polyploid.
Allopolyploidy in Crop Development
Allopolyploidy holds practical importance in agriculture and plant breeding, enabling the creation of new, improved crop varieties. Breeders can intentionally induce allopolyploidy to combine desirable traits from different species into a single, fertile plant. This process allows for the integration of characteristics like disease resistance, increased yield, or improved nutritional value from various ancestral lines.
Many familiar and economically significant crop species are natural allopolyploids. Bread wheat ( Triticum aestivum ), for example, is a hexaploid derived from three different ancestral species. This complex genetic background contributes to its adaptability and grain quality. Similarly, cotton (Gossypium hirsutum) is an allotetraploid, combining genomes from two distinct wild cotton species, which contributes to its strong and abundant fibers.
Rapeseed, also known as canola (Brassica napus), is another allopolyploid crop formed from the hybridization and chromosome doubling of Brassica rapa and Brassica oleracea. This allopolyploid nature provides it with a broader genetic base, enhancing its oil production and resilience. Tobacco (Nicotiana tabacum) also exemplifies an allopolyploid, resulting from the hybridization of two wild Nicotiana species, which led to its unique chemical composition. The intentional manipulation of allopolyploidy continues to be a tool for developing crops with enhanced productivity and desirable characteristics.