Genetics pp. 218-222 (2008-2009)
P. K. Gupta

Structural Changes in Chromosomes

Variations in the structure and number of chromosomes have been observed in natural populations and could also be produced artificially in a variety of organisms. These variations have been extensively studied and can be due to either structural changes or numerical changes. This chapter will be devoted to structural changes in chromosomes, and the next chapter will deal with numerical changes.

Structural changes can be of following types:
(i) deficiency which involves loss of a part of chromosome, (ii) duplication, which involves addition of a part of chromosome, (iii) inversion, which involves a reverse order of the genes in a part of chromosome, and (iv) translocatlon, which involves exchange of segments between non-homologous chromosomes. These structural changes are diagrammatically represented in Figure 19.1, where two non-homologous chromosomes from the complete set arc shown. Structural abnormalities may be found in both the homologous chromosomes of a pair, or in only one of theme When both homologous chromosomes are involved, these are called structural homozygotes e.g., deficiency homozygote, duplication homozygote, etc. If only one chromosome is involved, this will be called a structural heterozygote. The constitutions of a translocation heterozygote and that of a translocation homozygote are shown in Figure 19.2. In this chapter, a brief and elementary account of different structural changes is presented. For a more detailed account of this subject, reader are encouraged to consult author's advanced book 'Cytogenetics'.

Deficiencies
Deficiency is due to loss of a part of a chromosome. Smaller deficiencies, present in heterozygous condition (only on. one of the two homologous chromosomes), can be tolerated by an organism. Such individuals at meiosis will form a loop in a bivalent that can be observed at pachytene stage (Fig. 19.3) Loops can also be observed in salivary gland chromosomes of Drosophila which are found in a permanent state of pairing, so that even small: deficiencies could be detected in these chromosomes (Fig. 19.4). Deficiencies have an effect on inheritance also. In presence of a deficiency, a recessive allele will behave like a dominant allele (pseudodominance). This principle of pseudodominance exhibited by deficiency heterozygotes has been utilized for location of genes on specific chromosomes in Drosophila, maize and other organisms. L. J. Stadler, who was a pioneer in radiation work in plants devised a method where a homozygous recessive stock was pollinated by irradiated pollen from dominant stock, so that if irradiation induced a deletion, recessive allele will express due to pseudodominance.

As shown in Figure 19.5, if homozygote abc is pollinated by ABC, heterozygous F1 (ABC/abc) will be produced expressing only dominant characters. If pollen with dominant alleles ABC is irradiated, a deletion may be induced leading to expression of pseudodominance by one or more recessive alleles. If meiosis at pachytene is examined in such a deficiency heterozygote, presence of loop will indicate location of gene. Several genes were located on different chromosomes of maize and tomato, utilizing deficiencies. In Drosophila also deficiencies were recorded particularly on X-chromosomes in regions of genes w (white eye), fa (facet eye) and v (vermilion coloured eye). Deficiencies have also been recorded in waltzing mice in region of gene v inducing nervous abnormality. In human being, a deficiency was discovered, which was associated with cat like-cry so that the child carrying this deficiency had a cat like cry and also had microcephally (small head and low mental faculty). This deficiency was found in a segment of chromosome 5.

Duplications
Duplications are obtained due to addition of a part of a chromosome. If duplication is present only on one of the two homologous chromosomes, at meiosis (pachytene), cytological observations characteristic of deficiency will be obtained in duplication also (Fig. 19.6). Duplication of a chromosome segment, may be brought 'about by addition at any of the following positions: (i) in adjacent region (Fig. 19.7a), (ii) at a displaced position of the same arm (Fig. 19.7b), (iii) on the different arm of the same chromosome (Fig. 19.7c) or (iv) on a different chromosome (Fig. 19.7d). Sometimes the duplication may be found as a reverse repeat (Fig. 19.7e).

One of the classical examples of duplication in Drosophila is Bar eye (Fig. 19.8). Bar eye is a character, where eyes are narrower as compared to normal eye shape. This phenotypic character is due to duplication for a part of a chromosome. By the study of giant salivary gland chromosomes, it could be demonstrated that 'Bar' character was due to a duplication in region 16A of X-chromosome. Barred eyes will have slightly different phenotype in heterozygous and homozygous individuals (Fig. 19.9). Barred individuals (16A 16A) gave rise to ultrabar (16A 16A 16A) and normal wild type (16A) due to unequal crossing over (Fig. 19.10).

Some other duplications known in Drosophila lead to following phenotypic effects: (i) a reverse repeal in chromosome 4 causes eyeless dominant (Ey); (ii) a tandem duplication in chromosome 3 causes confluens (CO) resulting in thickened veins, and (iii) another duplication causes hairy wing (Hw).

Translocations
Translocations is a broad term including all types of unilateral or bilateral transfer of chromosome segments from one chromosome to another. An important class of translocations having evolutionary significance is known as reciprocal translocations or segmental Interchanges, which involve mutual exchange of chromosome segments between two pairs of non-homologous chromosomes (Fig. 19.11)

Cytology of a translocation heterozygote
If a translocation is present in one of the two sets of chromosomes, this will be a translocation heterozygote. In such a plant, normal pairing into bivalents will not be possible among chromosomes involved in translocation. Due to pairing between homologous segments of chromosomes, a cross-shaped (+) figure involving four chromosomes will be observed at pachytene (Fig., 19.11). This ring of four chromosomes at metaphase I can have one of the following three orientations:

Alternate. In alternate orientation, alternate chromosomes will be oriented towards the same pole. In other words, adjacent chromosomes will orient towards opposite poles. This will be possible by formation of a figure of eight (Fig. 19.11).

Adjacent I. in adjacent I orientation, adjacent chromosomes having non- homologous centromeres will orient towards the same pole. In other words, chromosomes having homologous centromeres will orient towards opposite poles. A ring of four chromosomes will be observed.

Adjacent II. In adjacent II orientation, adjacent chromosomes having homologous centromeres will orient towards the same pole. A ring of four chromosomes will be observed.

As shown in Figure 19.11, alternate disjunctions will give functional gametes. Adjacent I and adjacent II disjunctions will form gametes, which would carry duplications or deficiencies and as a result would be non-functional or sterile. Therefore, in a plant having a translocation in heterozygous condition, there will be considerable pollen sterility. A ring of four chromosomes, as described ahoy; is found under conditions when a single interchange is found. If two interchanges are involving three non-homologous chromosomes, a ring of six chromosomes is found, and the size of ring can increase with additional interchanges. More than one ring can also be found if two or more interchanges are independently found, each involving two different non-homologous chromosomes.

The first case of translocation was found in Oenothera, which was originally described as a mutation by de Vries while working for his Mutation Theory. Oenothera, Tradescantia and Rhoeo are such cases, where translocations in heterozygous condition are frequently found in nature. In many other crop plants they have been artificially induced.

Breeding behaviour of a translocation heterozygote
Presence of translocation heterozygosity can be detected by presence of semi- sterility and low seed act. This can then be confirmed at meiosis by quadrivalent formation. As shown above only two types of functional gametes are formed which result from alternate disjunction. The functional gametes will give rise to three kinds of progeny (Fig. 19.12) namely: (i) normal, (ii) translocation heterozygote, and (iii) translocation homozygote. These three types would be obtained in 1:2:1 ratio.


Interchange beterozygosity in Oenothera
Subgenus Euoenothera of genus Oenothera has been studied during 1920-1930 and cytogenetic structure leading to evolution in this group was examined. This group has 2n=14 and all 7 chromosomes of a haploid complement have median centromeres. Different species in the subgenus Euoenothera, can be classified in three groups: (i) First group is represented by species showing bivalents or small rings at meiosis (e.g. O. hookeri, O. grandiflora, O. argillicola). (ii) Second group is represented by species forming rings of various sizes at meiosis indicating the presence of interchanges. These rings are not permanent but are maintained due to their superiority in adaptive value (e.g. O. irrigua). (iii) The third group is represented by those having permanent translocation heterozygosity involving all chromosomes, so that a ring of 14 chromosomes is regularly formed (e.g. O biennis, O. strigosa, O. parviflora). In O. lamarckiana, a ring of only 12 instead of a ring of 14 chromosomes is observed (Fig. 19.13). These three groups also differ in phenotypes like flower size, etc. and can be identified. The members of third category behave like pure lines and are actually permanent heterozygotes.

Balanced lethals and gametic complexes: permanent hybridity in Oenothera. Permanent hybridity in some species of Oenothera is maintained due to operation of a balanced lethal system, which may function due to gametic lethality or zygotic lethality. Since complete rings are formed and alternate disjunction is a rule, only two types of gametes are formed showing complete linkage between 7 chromosomes. The gametic and zygotic lethality leads to survival of only heterozygotes (Fig. 19.14). It may be noticed that in gametic lethality, only one of the two types of gametes will function on the male side, the other type being functional on the female side, thus giving rise to only one type of progeny, which will be heterozygous. In zygotic lethality on the other hand, both the types of gametes will function on male as well as on female side, but the homozygote progeny due to recessive lethal genes will not survive (Fig. 19.14).

Inversions

An inversion is produced when there are two breaks in a chromosome and the intercalary segment reunites in reverse order i.e. the segment rotates at 180°. Let us imagine that a chromosome 1-2.3-4-5-6-7-8 gives rise to another chromosome having the order 1-2-7-6-5-4-3-8. The segment 3-4-5-6-7 has rotated here at 180° giving an inverted order of genes 7-6-5-4-3. A similar hypothetical example using a chromosome ABCDEF has been shown in Figure 19.15, where due to coiling, breaks occur between B and C as well as between D and E. Reunion at broken ends may lead to inversion of the segment CD into DC.

The inversion can be of two types: (i) paracentric inversion, and (ii) pericentric inversion. Paracentric inversions arc those inversions, where inverted segment does not include centromere. On the other hand, in a pericentric inversion, inverted segment includes centromere. In order to remember these terms and their meaning, one should bear in mind that pericentric means surrounding the centromere or on the periphery of centromere.

Cytology of Inversions

Due to an inverted segment in one of the two homologous chromosomes, the normal kind of pairing is not possible in an inversion heterozygote. In order to enable pairing of homologous segments, a shape of loop is formed by each of the two chromosomes as shown in Figures 19.16 and 19.20. This kind of configuration will be observed both in paracentric as well as in pericentric inversions. As will be observed, the products of crossing over and the subsequent stages of meiosis will differ in these two kinds of inversions.

Paracentric Inversion. A single crossing over or an odd number of crossovers in inverted region will result into formation of a dicentric chromosome (having two centromeres) and an acentric chromosome (with no centromere). Of the remaining two chromatids, one will be normal and the other will carry the inversion (Fig. 19.16). The dicentric chromatid and acentric chromatid will be observed at anaphase I in the form of a bridge and a fragment (Fig. 19.17). Double crossovers and crossovers within and outside inversion will give various kinds of deficiencies and duplications (Fig. 19.18). These will also give rise to a variety of characteristic configurations at anaphase I and anaphase II (Fig. 19.19; for details consult author's advanced text book Cytogenetics).

Pericentric inversion. In a pericentric inversion (where centromere is present within the inverted segment), the pachytene configuration observed is similar to the one described above for paracentric inversion (Fig. 19.16). However, the products of crossing over and configurations at subsequent stages of meiosis differ. In this case, two of the four chromatids resulting after meiosis will have deficiencies and duplications. However, unlike paracentric inversion, no dicentric bridge or acentric fragment will be observed (Fig. 19.20). Consequently, at anaphase I, no bridge or fragment will be seen.

However, in pericentric inversion, if two breaks are not situated equidistant from the centromere, this will result in a change in shape of the chromosome. For instance, a metacentric chromosome (with centromere in the centre) may become submetacentric and vice versa (Fig. 19.21).

Genetic consequences of inversion

As discussed in the preceding section on cytology, among four chromatids resulting after crossing over, the two chromatids resulting from crossing over would have deficiencies and duplications. The gametes having these chromosomes will not function. Therefore, there should be considerable gametic or zygotic lethality. In plants, there will be sufficient pollen sterility. (For details see author's book Cytogenetics). However, since the products of single crossover will not function and the only crossovers recovered will be double crossovers, the observed frequency of recombination between any two genes in question will be considerably reduced. Due to this reason, inversions, are often called crossover suppressors. This reduction in crossing over is not the actual reduction in cytological crossing over, but is the result of lack of recovery of the products of single crossovers. This property of inversions has been utilized in the production of ClB stock, used by H.J. Muller for the detection of sex linked lethal mutations (Chapter 21).

Overlapping Inversions

Sometimes a second inversion is induced in a chromosome which already has one inversion. This results in an overlapping inversion, if the segments involved in first and second inversions contain a common region. The gene orders and meiotic configurations found in an inversion heterozygote of this type are shown in Figure 19.22.

Inversions in Drosophila populations

Inversions are known to have played a significant role in evolution of different species and races of Drosophila. This knowledge is particularly available in Drosophila due to the ease of identification of inversions in salivary gland chromosomes. They also occur in plants, but can not be so easily worked out in the absence of giant chromosomes. In Drosophila, however, it is obvious that inversions occurred spontaneously in nature and became established in populations due to the adaptive benefit they conferred. Due to adaptive value, these inversions are restricted to definite localities and the different races actually derived their names after these localities. Th. Dobzhansky and A.H. Sturtevant even derived evolutionary relationships between races on the basis of overlapping inversions. These relationships are shown in the form of a tree in Figure 19.23.