B-chromosomes and plasticity of the species

Population studies of widespread species with B chromosomes have not yet received sufficient attention. A number of works carried out with such species to assess the variability of both the frequency of individuals with B-chromosomes and the variability of the average number of B-chromosomes in the population indicate the possibility of the influence of B-chromosomes on the plasticity of the species.
In particular, A. schoenoprasum, which spreads even into the Arctic up to 750 N. (Novaya Zemlya) can carry up to 20 B chromosomes in one cell under these conditions. The high ecological plasticity of the species noted for A. schoenoprasum is accompanied by an increased frequency of occurrence of B chromosomes and an increased number of them in this plant species (Bougourd and Prowman, 1966).
In maize populations polymorphic in B chromosomes from Northern Argentina, a positive and statistically significant correlation was found between the average number of B chromosomes and plant height (Ricci and Silva, 2007).
The frequency of plants with B chromosomes in natural populations is usually low, but it increases under unfavorable growing conditions. Extra chromosomes are more often found in plants growing in extreme conditions. It is believed that B chromosomes can increase the adaptive potential of plants. Forms that have them are more resistant, for example, to drought and low temperatures (Moshkovich, 1979).

Among almost 20 species of Apodemus (Sylvaemus) mice, B chromosomes were identified in 6 species. Among them, two species with significant population variability of B chromosomes have the widest range. First of all, this is the East Asian mouse A. peninsulae with its huge range in the eastern part of the Palearctic. The range of another mouse species, the yellow-throated S. flavicollis, occupies a significant part of Europe and reaches the Volga River basin in Eastern Europe. In this species, population variability was revealed, mainly in 1-3 small acrocentric B chromosomes, and their origin is most likely associated with duplications of small acrocentric autosomes and their heterochromatization. Among the Apodemus (Sylvaemus) mice, there is also a third widespread species – the field mouse A. agrarius (which lives in the Far East and in the territory from Lake Baikal to Central Europe), but in this species only rare individuals were found to have a very small acrocentric B-chromo catfish (Kartavtseva, 2002). The plasticity of species, even within the same genus, can probably be provided by various mechanisms associated with molecular and genetic variability. Moreover, the origin, DNA structure, and the number of B chromosomes in these three species are completely different (Rubtsov et al., 2004).
The maximum range of population variability of B chromosomes was observed in A. peninsulae in Siberia, where, under extreme habitat conditions for the species, the number of B chromosomes varies from 1 to 30 (Borisov et al., 2010). In more favorable habitats in Primorye, Korea and China, the variation in the numbers and morphotypes of B chromosomes is much less. In Primorye and Khabarovsk Territory, mice without B chromosomes are often found (up to 25%). In mice of the Far East, their variability reaches 0 – 6 only in rare individuals (Kartavtseva, 2002).

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