last  page PLNT3140 Introductory Cytogenetics
Lecture 21, part 1 of 4
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November  20, 2014

CHANGES IN CHROMOSOME NUMBER


REFERENCES:
Singh, R.J. 1993. Plant Cytogenetics CRC Press Inc. Chapter 6B.

Otto SP, Whitton J (2000) Polyploid incidence and evolution. Ann. Rev. Genet. 34:401-437.


Learning checklist:

1. Be able to define autopolyploidy and allopolyploidy, and know the terms for the different types of ploidies eg. autotriploidy, allotetraploidy.

2. Know how to use the terms "n" and "x" for describing chromosome number in a species, and with reference to aneuploidies.
3. Be able to explain why aneuploids or triploids have decreased fertility.
4. Understand how crossing over in polyploids can lead to the formation of chromosome chains at diakinesis.
5. Be able to explain how polyploidy influences evolution.
6. Understand why chromosome doubling is critical for the establishment of an allopolyploid species.



A. The concepts of ploidy and basic genomes

Euploidy vs Aneuploidy
Euploid: Individual with one complete chromosome set or with multiples of the basic number of chromosomes characteristic of the species.
Aneuploid: Individual with one or more whole chromomsomes in addition to the euploid complement or missing from the euploid complement (lecture 21).

The n vs x concept

n: gametic number of chromosomes (akin to the complexity of the genome)    

2n: zygotic or somatic number of chromosomes

x: basic number of chromosomes, with respect to a common ancestral species ie. x is the "original" set of chromosomes, from which contemporary species are derived.

Example:

Bread wheat (Triticum aestivum) is a hexaploid having a total of   42 chromosomes in its somatic cells. What are n and x?
2n =42; therefore n = 21
6x (hexa)= 42; therefore x = 7
The genomic formula for breadwheat is 2n = 6x = 42

Euploidy
Ploidy level and origin of ploidy

Ploidy level: Refer to the number of basic chromosome sets. Monoploid (basic, 1x), diploid(2x), triploid (3x), ...hexaploid (6x), ...octoploid(8x). Individual with three or more sets are polyploids.

Polyploids are prominent in the plant kingdom,making up 30-35% of Angiosperm species,  75% of the the Graminae species. Polyploids have advantages in a wider ecological range of tolerances.  In seed plants, the endosperm is normally triploid. Therefore, seed plants must have intrinsic mechanisms for tolerating triploidy.

Polyploids are less common in animal species, particularly among mammals. However, polyploidy is often seen in amphibians and salmonid fish.  

Origin of ploidy : There are two main categories depending on the origin of the chromosome sets that make-up the polyploid



B. Autopolyploidy

Autopolyploidy in plant breeding
     The benefits of polyploid breeding include increased size of plant organs- roots, leaves, flowers fruits and seeds. Chemical characteristics change: for example tetraploid maize has 40% more vitamin A content than its diploid counterpart. In polyploid sugar beets, larger roots are desirable for total sugar harvest per hectare although sugar content decreases as root size increases. The autopolyploids differ in phenotype "quantitatively" from their parents. The disadvantages lie in the reduced pollen production and increased seed sterility in the autopolyploids. Some crops such as triploid potatoes are easily reproduced vegetatively and are therefore very competitive agronomically.

1. Autotriploidy

Autotriploid:  Individual possessing three basic sets of homologous chromosomes (3x).

a. Origin of autotriploids

Autotriploids arise from the fertilization of an unreduced egg (2n) by a normally reduced male gamete (n) of an originally diploid species. Autotriploidy can also be induced. Autotriploids have been identified in at least 68 different plant genera. They are phenotypically more vigourous but are not competitive at reproduction due to a high level of ovule abortion. This is advantageous in banana and watermelon where seedless is prefered.

Triploids are a good source of trisomics.

The problem with triploids

Triploid zygotes can produce many possible gametes, because each gamete can get either one or two copies of each chromsome, in many possible combinations. Consider a species with only two chromosomes. Even in this simple case, only one out of four possible gametes will have the normal haploid complement of chromosomes. Aneuploid gametes are usually deleterious.  In a mating with a normal  haploid gamete, the tetraploid gamete would produce a triploid, while a 2n + 2n mating would give a tetraploid, which may or may not be deleterious (more discussion below).



The number of  possible combinations of chromosomes increases rapidly as the number of chromosomes increases. Therefore, the number of balanced gametes drops rapidly for genomes with larger numbers of chromosomes.

Example: Impact of triploidy on seed set.

Table 6.15. Summary of the Total Material Studied from 39 Barley Cultivars
Number of spikes = 18,719;  Total number of seeds = 381,563
Seed size groups 1000 kernel weight (g) Number of seeds Germination 
(%)
Number of plants Triploid 
(%)
Diploid

2n = 14

Triploid

2n = 21

Aneuploid
A 41.2 365,645  - - - - -
B 13.7 7,480  73.9 5364 158 7 2.86
C 3.5 8,438  2.4 182 21 1 10.29
from Sandfaer, J.1975. Hereditas 80:149-153. 
Seed size group A represent the fully developed seeds (95.8%). They were assumed to correspond to normal diploid barley seeds. Seed size group B is the shrivelled seeds (2.2%) where 158/5529 seeds were triploids. Seed size group C were the "empty" seeds (2.2%). A total of 21/204 seeds were of triploid origin. Triploid seeds are not as vigourous and competitive (low germination, small seeds) as the diploid parent.

b. Cytological behaviour in autotriploids


To start, let's remember what normal pairing looks like:



Autotriploidy is comparable to multiple primary trisomics. The three homologous chromosomes can form trivalents as well as bivalents with univalents.        

Trivalent association is more common with longer chromosomes. 

Any region can pair, but only two chromosomes can pair in any one region.  If two of three homologues pair along their entire length, trivalents cannot form because one chromosome will be left completely unpaired, resulting in a bivalent and a univalent. If all three chromosomes are involved, chiasma formation may result in paired segments.


At diakinesis, four possible configurations can be seen depending on the crossover that occured at pachynema: a. chain (V-shaped),b. ring-rod (frying pan), c. triple arc (bird-cage)and d. Y -shaped.

Figure 6.20
. A, possible trivalent configurations at diakinesis in an autotriploid, based on crossing over and pachytene chromosome pairing.(Redrawn from Kuspira et al., 1986. Can. J. Genet. Cytol.28:867-887.)


              
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