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Lecture 20, part 1 of 5
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November 21, 2017

CHANGES IN CHROMOSOME STRUCTURE


REFERENCES

SINGH, R.J. 1993. Plant Cytogenetics. Chapter 6A.pp.83-109.
SCHULZ-SCHAEFFER, J.1980. Cytogenetics. Plants. Animals. Humans.Springer-Verlag NY.




Key take home lesson: Due to chromosome pairing, inversions and translocations are important evolutionary mechanisms:
  1. By driving speciation due to reduced viability of hybrid progeny
  2. By creating new chromosomal arrangements

III. TRANSLOCATIONS

A. Types of translocations

B. Identification of translocations

C. Effects of translocations on gametes

D. Role of translocations in genome evolution

IV. INVERSIONS

A. Paracentric inversions

B. Pericentric inversions

V. EVOLUTION AND SPECIATION




"Nothing in biology makes sense except in the light of evolution" - Theososius Dobzhansky

When I first began teaching this course in 1990, it was targeted at plant breeders. The course was therefore highly practical in it's spin on biology. Plant breeders were interested in the fact that changes could occur in chromosome structure primarily with respect to the deleterious effects of gross structural changes on fertility. Therefore, the material was presented primarily as a set of facts to be learned, and mechanisms to be understood. The terms "evolution" and "speciation" were used sparingly, if at all.

As I began to understand these mechanisms better myself, I began to realize the profound implications that chromosomal changes had on speciation, and therefore, on the larger process of evolution. Each time I reexamined another piece of this subject matter in an evolutionary context, it suddenly clicked. It now made a great deal of sense. In my mind, a dry and convoluted area of genetics suddenly became one of the great keys to my understanding of evolution.

A. Types of translocations

Translocations: Translocations are the result of the reciprocal exchange of terminal segments of non-homologous chromosomes.   

Heterozygote for a reciprocal translocation




Image displayed by hypertext link to
http://cnx.org/content/m44483/latest/Figure_13_03_09.jpg

Origin:

B. Identification of translocations

1. Normal pairing

It's best to start by reviewing  chromosome pairing at meiosis. At pachytene, homologous chromosomes are completely paired. After desynapsis during diplotene, chromosomes remained paired at their termini. During metaphase I, sister chromatids are still together, but kinetochores have separated.

2. Pairing of translocation heterozygotes at pachytene

One way or the other, homologous sequences will find each other at pachytene, regardless of which chromosomes they happen to be on.

When a reciprocal translocation is in the heterozygous state, the four translocated chromosomes must pair together, bending in such a way as to bring together all homologous regions, regardless of which chromosome they are on. For reciprocal translocations, a cross-shaped pairing, referred to as a quadruple,  is required to bring together all homologous regions.

Note: When a translocated chromosome is homozygous, no aberrant pairing occurs!


3. Pairing of translocation heterozygotes at meiotic metaphase I

At metaphase I, several types of configuration are possible, depending on the segregation of the kinetechores. These orientations are given specific designations and the consequence for the translocation heterozygote are distinct. 


Alternate: both normal chromosomes go to one pole, both translocated chromosomes go to the other pole. Result: balanced gametes.

Homologous kinetechores move to opposite poles. Alternate configurations result in balanced gametes, since each daughter cell gets  a complete set of chromosomes. (Remember that in meiosis II, each pair of sister chromatids shown at right will segregate to opposite daughter cells.


 


DEMO: translocation_demo.obj
(You can save this file and open it on your Unix account using the TGIF drawing program.)


Adjacent : One normal and one translocated chromosome move to each pole. Result: unbalanced gametes

When sister chromatids for a translocated chromosome segregate to opposite daughter cells, both daughter cells are missing part of one chromosome and have a duplication for another part.



Adjacent-1: One normal and one translocated chromosome move to each pole (normal behavior for centromeres)

Adjacent-2: Both homologous centromeres move to the same pole (rare, abnormal behavior for centromeres)

In co-orientation configuration, the chromosomes are distributed in equal numbers to opposite poles. In adjacent-1, the homologous kinetechores move to opposite poles.
 

Non-coorientation : two non-co-oriented chromosomes are stretched across the metaphase plate and not attached to the poles. Result: unbalanced gametes

At anaphase, the segregation is 1 cooriented and 2 non-cooriented chromosomes migrate to one pole, and 1 cooriented chromosome to the other pole producing a 3:1 chromosome segregation which results in aneuploid progeny-trisomic and monosomic.


 


 
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