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The mysterious life story of supernumerary chromosomes: How are they formed in the biological world?
In the wild populations of various animals, plants and fungi, in addition to normal karyotypes, there are also B chromosomes (also known as superchromosomes). These chromosomes are not essential for the life of the species and are completely missing in some individuals. Therefore, a certain population may contain different numbers of B chromosomes, showing variations such as 0, 1, 2, 3, etc. These B chromosomes are different from marker chromosomes or extra copies of normal chromosomes, and their formation and evolution remain a mystery.
"Supernumerary chromosomes can be regarded as a special category of genetic polymorphisms. Due to various accumulation mechanisms, these chromosomes do not follow ordinary Mendelian laws of inheritance."
The evolutionary origin of this type of chromosome is still unclear, but it is speculated that it may originate from the heterochromatic part of normal chromosomes. Taking next-generation gene sequencing as an example, it has been found that the B chromosome in rye is actually a fusion of the rye A chromosome. The B chromosome of some African cichlid fish (Haplochromis latifasciatus) has also been confirmed to be derived from the recombination of the normal A chromosome.
Although most B chromosomes are primarily heterochromatic, that is, they have limited coding functions, some contain considerable euchromosomal segments, such as the B chromosome of maize. In addition, there is diversity in the functionality of B chromosomes. In some cases, they may act as selfish genetic elements, but they may also provide positive advantages in adapting to the environment. For example, the British grasshopper (Myrmeleotettix maculatus) possesses two types of structural B chromosomes. When they appear in dry tropical environments, their numbers increase significantly, but they are relatively rare in wet and cold environments. This also illustrates the correlation between supernumerary chromosomes and specific habitats.
"It is currently known that supernumerary chromosomes will have adverse effects during pollen reproduction, but beneficial effects are also common in specific environments."
In the fungal world, chromosomal diversity is widespread. Even different isolates of the same species often have different chromosome numbers, and these extra chromosomes are not necessary for normal growth in culture. Therefore, supernumerary chromosomes are called conditionally dispensable chromosomes because they may confer a selective advantage in specific environments. Although these supernumerary chromosomes do not carry genes required for basic growth, they may have some functional significance.
“For example, the supernumerary chromosomes of the pea pathogen Haematonectria haematococca carry genes important for its pathogenicity.”
The DNA in this supernumerary chromosome was found to encode a group of enzymes that metabolize toxins secreted by the plant immune system. For the fungal pathogen Zymoseptoria tritici that affects wheat, it possesses as many as eight supply chromosomes, the highest supernumerary chromosome number ever observed in a fungus.
In the plant kingdom, B chromosomes are an important reflection of genetic diversity, and these supernumerary chromosomes are commonly observed in naturally pollinated flowering plant species. B chromosomes occur in different forms in different species, and their number varies among members of the same plant group. For example, the sister species Aegilops speltoides and Aegilops mutica possess copies of the B chromosome in their aerial tissues but lack these supernumerary chromosomes in the roots. The B chromosome of plants is significantly different from normal chromosomes in morphological structure and size, and is usually "non-homologous and smaller than the smallest A chromosome".
With the deepening of research, scientists have increasingly deepened their understanding of the formation mechanism and evolutionary process of B chromosomes, but what role do they play in the biological world?
