Polyploid Speciation Explained With Simple Examples

Speciation occurs due to various environmental reasons. While some species arise due to climatic adaptations, few others arise due to the migration of a species into a new geographic habitat. Such adaptations and migrations give rise to an isolating barrier between the two species. Hence, it is categorized into four distinct types based on the degree of isolation between the speciating populations. They are as follows.

• Allopatric – The population is divided due to geographical change in the habitat, and the isolated populations undergo phenotypic and genotypic divergence to produce two distinct reproductively isolated species.
• Peripatric – The population undergoes a founder effect, where a few individuals migrate to another niche, resulting in a distinct, isolated population.
• Parapatric – The populations are only partially separated, but the organisms still develop reproductive isolation as interbreeding between them is somehow hindered.
• Sympatric – The isolating event is genetic in nature. Polymorphisms in the population’s genome lead to the development of a new species that is reproductively incompatible with its predecessor.

Sympatric speciation is the only type which is caused due to genetic reasons. Mutations and polymorphisms in the genome of a species cause the malfunctioning of cellular processes, resulting in the production of dysfunctional gametes, which when fertilized, give rise to progeny that are genotypically and/or phenotypically dissimilar. Since the gamete themselves are abnormal, the zygote hybrid that is formed does not have the normal number of chromosomes. This anomaly in the number of chromosomes renders the progeny unable to mate with the parental species. Also, the extra/deficit genetic elements alter the genotype and phenotype of the progeny, and hence a new species is produced.

Sympatric speciation occurs via hybridization or via polyploidy. In case of hybridization, the progeny that is produced is just a result of the crossing of two different species of the same genus, and shows the same ploidy level as the parental species. This prevents reproductive isolation. Ploidy refers to the number of sets of chromosomes present in the nucleus of a cell of an individual. In case of polyploidy, the progeny that is produced bears a ploidy level that is different from that of the parental species, and hence reproductive isolation is observed.

Polyploidy refers to the scenario where a cell has more than two copies of its chromosomes. A cell that possess only one set is called haploid, two sets is called diploid, three sets is triploid, and four sets is called tetraploid. However, in general, all cells with more than two sets are called polyploids. In the event that a diploid species gives rise to a polyploid species, an obvious consequence that is observed is that the resulting progeny is incapable of breeding with the diploid species.

This instantaneous reproductive isolation and the possession of a unique genotype and phenotype is the end result of polyploid speciation. This is observed more in case of plant and lower animals. In case of higher animals, polyploid individuals either prove fatal during embryonic birth itself or if they survive, exhibit debilitating health disorders. Polyploidy can occur in the progeny via two ways and they are named, autopolyploidy and allopolyploidy.

❍ In 1982, Gerhardt identified tetraploid populations of Hyla versicolorarising from breeding of diploid populations of Hyla chrysoscelis. He observed that these frogs could proliferate only by breeding with another with a similar ploidy. If a diploid and tetraploid frog did mate, it produced a triploid offspring that perished during the larval developmental stages. The rare individuals that survived were completely sterile.

❍ In the case of Cnemidophorus or whiptail lizards, progeny are produced via polyploidy. These lizards exhibit asexual parthenogenesis, through which they derive clonal reproduction of themselves, in turn making the male obsolete. Hence only females of this species are seen. The process of parthenogenesis involves inducing polyploidy in order to produce diploid eggs that later give rise to clonal progeny. They are otherwise sterile.

❍ In 1950, Owenby proved that the flowering plant Tragopogon miscelluswas produced due to the hybridization of T. dubius and T. pratensis.

❍ A species of hemp-nettle, Galeopsis tetrahit, is an example of polyploid speciation of two other species, G. pubescens and G. speciosa.

❍ In the genus Brassica, Frandsen observed that B. Carinata is produced by hybridizing B. nigra and B. oleracea; B. juncea by hybridizing B. nigraand B. campestris, and B. napus by hybridizing B. oleracea and B. campestris.

❍ Triticum aestivum, wheat, is a result of polyploid crossing of T. monococcum, wild Triticum, and T. turgidum.

In recent times, intentional and artificial speciation can be induced via the use of selective breeding practices coupled with genetic engineering techniques. The problem of sterility can also be resolved by growing them in appropriate conditions in the laboratory and ensuring their reproduction via suitable techniques.