The following points highlight the two main ways of representing chromosome set of species. The ways are: 1. Karyogram 2. Idiogram or Idiotype.
Representing Chromosome: Way # 1. Karyogram:
Photomicrographs of the chromosomes of a single representative somatic metaphase cell are clipped out and arranged in homologous pairs according to their size. If the chromosomes are small and there is difficulty in identifying the individual chromosomes, they are arranged in groups of similar chromosomes.
For example, in human, the 23 pairs of chromosomes had been divided into 7 groups represented by the letters from A to G (Denver System) and numbers (London System); the seven groups are, A (l, 2, 3), B, (4, 5), C (X, 6, 7, 8, 9, 10 11, 12), D (13, 14, 15), E (16, 17, 18), F (19, 20), and G (Y, 21, 22).
Thus the X chromosome is placed in the C group, while the Y chromosome is placed in the G group. However, it is now possible to unambiguously identify each of the 23 chromosomes, and even individual chromosome arms, with the help of chromosome banding.
Representing Chromosome: Way # 2. Idiogram or Idiotype:
It is the graphical representation of the karyotype (Fig. 6.5). Generally, the idiogram is prepared to show the haploid chromosome complement of a species; it is prepared from the measurement of somatic metaphase chromosomes.
Individual chromosomes must be identified for this purpose. There are techniques by which chromosomes or even specific chromosome segments can be identified. These techniques are fluorescent staining, pulse labelling, chromosome banding, and studying the tertiary constrictions and chromomeres.
Chromomere pattern can be studied easily and clearly in pacyhtene stage in many species and in polytene giant chromosomes of several members of Diptera.
Symmetry and Asymmetry of Karyotype:
Karyotypes may be symmetrical or asymmetrical; this concept was developed by Levitzky in 1931. When all the chromosomes of a species are of approximately the same size and have median or sub-median centromeres, the karyotype is said to be symmetrical (Fig. 6.5).
When the chromosomes of an individual differ in size and position of centromere, the karyotype is called asymmetrical or heterogeneous. The symmetrical karyotype represents a primitive state from which asymmetrical karyotypes have evolved through structural chromosome changes.
Pericentric inversions and unequal translocations change the position of centromere. Thus a metacentric chromosome may be converted into an acrocentric chromosome (Fig. 6.6, 15.5). However, reversion may also occur so that an acrocentric chromosome would become a meta- or sub-metacentric by the same process.
The term bimodal karyotype refers to an symmetrical karyotype that is composed of two distinct classes of chromosomes as determined from their size. Examples of such karyotypes are found in certain genera of Liliales, such as, Aloe, Gasteria, Yucca, Agave etc.
The species of Aloe and Gasteria have 7 pairs of chromosomes (x = 7) of which four are large and acrocentric, while three are short (Fig. 6.5). The species of Yucca and Agave have 30 chromosome pairs (x = 30) of which 5 are medium sized, strongly acrocentric chromosomes, while 25 are very small chromosomes.
The origin of bimodal karyotypes can be explained on the basis of pericentric inversions, unequal translocations and addition of centric fragments.
Karyotypic variations among different species of the same genus may be observed in several herbaceous genera possessing medium to large chromosomes in size.
A well known example is the genus Crepis (Compositae) where the degree of karyotype symmetry and chromosome number are negatively associated. C. kashmirica (x = 6), C. sibirica (x = 5), C. conyzaefolia (x = 4) and C. capillaris (x = 3) have larger chromosomes while C. mungieri (x = 6), C. leontodontoides (x = 5), C. suffreniana (x = 4) and C. fuliginosa (x = 3) have smaller chromosomes.
The species having smaller chromosomes exhibit a greater degree of karyotype asymmetry. However, in certain cases, such as, Clarkia and Cephalaira-Succisa, the degree of asymmetry increases with an increase in chromosome number. There are several factors which generate variation in the karyotype during evolution.
For a comparison of karyotypes of related species or genera, the following characteristics are considered:
(a) Absolute size of chromosomes:
Duplications cause differences in the absolute size of chromosomes.
(b) Centromere position:
The position of centromere changes due to unequal translocations and pericentric inversion in which the broken segments on the two sides of the centromere are not equal (Fig. 6.6, 15.5). A metacentric chromosome may be changed to become a sub-metacentric or sub-telocentric chromosome.
(c) Relative size chromosomes:
Segmental interchange involving translocation of unequal size is responsible for change in relative chromosome size. Two chromosomes of equal length may change into one smaller and one larger chromosomes (Fig. 6.6).
(d) Basic number:
Basic chromosome number may be reduced due to unequal translocation and accompanied with a loss of the centromere. Increase in basic chromosome number may occur by addition of centric fragments and translocation of essential gene loci to them (Fig. 6.7).
Example of reduction in basic chromosome number can be well understood by studying the multiple sex chromosomes. The XY mechanism evolved into XY1Y2 mechanism of sex determination by translocation between the sex chromosomes and an autosome pair, as in Rumex (plant) and certain insects.
The texas race of Rumex hastatulus has 2n = 10 chromosomes (8A + XY, ♂ and 8A + XX ♀). The North Carolina race of this species has 2n = 8 chromosomes in females (6A + XX) and 2n = 9 chromosomes in males (6A + XY1Y2).
During male meiosis, both arms of X chromosome pair with the two Y chromosomes, resulting in the X-Y1Y2segregation. The evolution of such karyotype is shown diagrammatically in Fig. 6.8.
The X1X2Y mechanism is considered to have evolved from the XO type by translocation between an autosome and the X chromosome, as in marrtids (Orthopteran insect). The two trans-located chromosomes become X1 and X2 while the non-trans-located homologue of the autosome pair becomes the “neo-Y chromosome” (Fig. 6.9).
The nee-Y chromosome is oriented towards one pole, while the Xt and X2 chromosomes are oriented towards the other-pole at metaphase I. However, there is no change in the chromosome number. The female possesses X1X1X2X2, while male possesses YX1X2 chromosomes in such condition.
(e) Number and position of satellites:
Location and size of the nucleolar organizer regions may differ.
(f) Heterochromatic regions:
Heterochromatic regions may be scattered or localized at different positions in the chromosomes.
Fig. 6.9 Diagram showing of X1X2Y mechanism of sex determination from OX type of male, by means of translocation between the X chromosome and an autosome (A). (Δ indicates the break position).