Chromosome 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 above, 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 set. 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 heterozygosity in Oenothera
Subgenus Euoenothera of genus Oenothera has been studied during 1920-1930 and cytogepetic 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).