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C. Effects of crossing-over on translocated chromosomes

The region between the kinetechores and the translocation is called the interstitial region. If no cross-overs occur in the interstitial region, alternate chromosome migration results in balanced gametes: one half containing the normal chromosomes 1 and 2, the other half with the 1' and 2' translocated but balanced chromosomes. The resultant meiotic products depend upon how the kinetochores segregate.

Crossover between centromere and translocation breakpoint.

unbalanced gametes


unbalanced gametes





Crossover after translocation breakpoint.

balanced gametes


balanced gametes

There is an approximate 1:1 ratio of alternate: adjacent arrangements of chromosomes in meiosis of an interchange heterozygote.

Consequences: In diploid plants, the presence of a heterozygous interchange produces approx. 50% pollen sterility. This is due to the 1:1 ratio between alternate (viable ) and adjacent (non-viable) orientations.

In polyploids, interchange heterozygotes do not show 50% pollen and seed sterility although the parents may differ by two interchanges. Male and female gametes containing Dp-Df in polyploid are as competitive as normal gametes due to the genetic compensation possible in polyploids.
 

D. Role of translocations in genome evolution

If a primitive species can be identified, chromosome pairing can be observed in crosses with related species to discover how chromosome structural changes have led to speciation.

Example: Secale rye

The cultivated species is Secale cereale rye and four wild species S. vavilovii, S.africanum, S.montanum, S. silvestre. All contain 2n=14 chromosomes.

Reciprocal translocations have contributed to the effective reproductive isolation of the wild and cultivated species. Translocations set limits on genetic exchange because of the resulting sterility. The chromsome pairing patterns observed in the F1 plants are shown in Figure 6.14 and 6.15.

1. By observing meiotic paring in interspecific hybrids, we can infer the translocation events that led to the divergence of species

Normal chromosomes should give the characteristic "D" shape appearance at meiosis. When chromosomes in a hybrid have undergone a reciprocal translocation, the translocated ends will be paired in the hybrids at the end of prophase I. The result is chains of chromosomes.


Figure 6.16. Meiotic metaphase-I configurations in Secale interspecific, F1 hybrids.  A, S. africanum x S. vavilovii (2 IV + 3II);  B, S. cereale x S. montanum (1VIII + 3II). (From R.J. Singh, unpublished results.)



The results of hybridizing diferent Secale species are summarized at right.

Because homologous arms pair at pachytene and are still attached after terminalization at diakinesis, chromosomes enter metaphase I in chains, in which homologous ends are linked.


Redrawn from Figure 6.14. Cytogenetic relationships among five major Secale species. Abbreviations: AFR, africanum ; CER, cereale; MON, montanum; SIL, silvestre ; VAV, vavilovii.  Crossability rate (%) in parentheses is from Khush (1962) and in parentheses with an asterisk is from Singh (1977).
 
Roman numerals: The number of chromosomes that pair as a chain.
Secale_species.gif  

2. The pairing relationships in parents and hybrids can be represented in a diagram.

Synteny in Secale species




Diagram of the chromosomes of four Secale species and their arrangements in corresponding interspecific hybrids. Arbitrary numbers are used to uniquely label each chromosome arm. Arm designations of species were derived from C-banding patterns of translocation complexes in the possible hybrid combinations. (Redrawn from Singh, R.J. and Röbbelen, G. 1977. Chromosoma (Berl.)59:217-225. Fig. 6.15.)

 

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Lecture 20, part 2 of 5
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