Evolution and Adaptation with Change of Chromosome Number

Viola

Jones (1928)
A detailed study of Viola species crosses and their chromosome constitution has been made by J. Clausen (1926). By crossing two varieties of Viola arvensis, each with 17 chromosomes, he has obtained a form with 16 chromosomes that is very different in appearance from the two original varieties, constant in its breeding behavior, and fertile.

From the cross of two species, Viola tricolor and V. arvensis, having 13 and 17 chromosomes, respectively, Clausen obtained an intermediate type with 14 chromosomes which possessed a character not found in any other Viola species. Furthermore, from another cross of these same two species he found a type with a higher chromosome number than either parental species, ranging from 21 to 23. Corresponding chromosomes from these two species are not always capable of conjugating. Sterile and abnormal plants are produced. There is an elimination of certain chromosomes so that recombination is not of the regular Mendelian type. In the progeny of this hybrid there is shown a tendency for the parental combination of genes, upon which the systematically important characters are dependent, to survive and maintain the types of the original species.

Clarkia

Lewis (1962)
... they differ markedly in chromosome composition: C. lingulata (n = 9) has an additional chromosome, not present in C. biloba (n = 8). which is homologous to parts of two chromosomes of C. biloba. In other words, part of the basic genome of C. biloba is duplicated in C. lingulata.

Humans (x=23) and Apes (x=24)

Lycoris aurea

Shu (2012)
Second, the chromosome numbers and karyotypes of L. aurea had great variation too, according to the number of chromosomes, the 10 populations also could be divided into five categories: (1) 2n = 12 = 10m +2T, Zhongshan botanical garden; (2) 2n = 14 = 8m +4T, Enshi Hubei, Jinfoshan, Chongqing and Laibin Guangxi; (3) 2n = 15 = 7m +8T, Jiangxi and Fujian; (4) 2n = 16 = 6m +10T, Mingshan and Emeishan Sichuan, Kunming Yunnan; (5) 2n = 18 = 4m +11t +3T, it might be a hybrid between L. aurea having 2n=8m+6T and L. radiate that having 2n=22t, this type was reported for the first time in Lycoris, only the population of Sangzhi Hunan had this karyotype. However, the variation was not only in chromosome’s number, because the karyotypes which had the same number of chromosome were also have variation in the morphology and arm ratio, all of these could be used to distinguish between the different populations of L. aurea.

Purpura (Nucella) lapillus.

Ford (1965)
69 Heterozygous advantage is evident also when the control of polymorphism involves whole chromosomes (pp. 23-4). 69 An instance of the kind, worked out in considerable detail both on the ecological and cytological side, is provided by the marine Gastropod Purpura (Nucella) lapillus.

Staiger (1954, 1957) has shown that two basic forms of this species exist, with n=13 and n=18 chromosomes. All the possible intermediates between them, amounting to 243, can be produced and seem to be viable and inter-fertile whether heterozygous or homozygous. That situation is the result of a delicate cytological adjustment. In the first place, all individuals possess eight chromosome-pairs that remain constant in structure. Furthermore, the n=13 set includes five V-shaped metacentrics each of which can break into two rod-shaped acrocentrics. Evidently the centromere is here a super-gene through which the metacentrics split.

Staiger finds that these different chromosome types are broadly adapted to the environment. The 2n=26 form inhabits exposed sites and that with 2n=36 is found in sheltered ones, while those with intermediate chromosome numbers colonize habitats that are intermediate also relative to wave action.

This distinction, though related to micro-habitats, is effective also upon a larger scale. In the Roscoff region, Brittany, Staiger showed that the 2n=26 and 2n=36 forms exist in appropriate conditions and form heterozygous colonies in intermediate places. Yet 15 kilometres further east, where the coast is relatively uniform and exposed, the 2n=36 type is absent and both exposed and intermediate localities maintain a pure 2n=26 population. Thus regional differences influence chromosome-numbers in habitats of an intermediate.

The shell-structure of Purpura lapillus is affected both by the cytological situation and the environment. The shells are largest and have the thinnest walls in the two pure populations (2n=26 and 36); they are the shortest and thickest in the most heterozygous ones. This is not a function merely of the amount of material used, for shells of identical dimensions have thicker walls when chromosomally heterozygous than when homozygous. Moreover, owing to the correlation between structure and habitat just mentioned, the shell-thickness decreases as we pass from places with an intermediate to an extreme degree of exposure.