Autotriploids in meiosis

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An autotriploid has three sets of chromosomes, all with the same karyotype. Therefore, each chromosome in a triploid occurs in groups of 3 homologues - chromosome is trisomic

During meiosis the groups of three homologous chromosomes synapse, two chromosomes at one place at a time.

Synaptic possibilities of one homologous group of three

With submetacentric chromosomes the most likely pairing association is a trivalent. One bivalent and a monovalent (univalent) or 3 monovalents are less likely. All 3 paring associations are unsaturated, containing chromosome segments that are not paired.

Chromosome associations of a triploid of 2n = 9 = 3x

When viewing one meiocyte, a number of pairing associations are possible for the three groups of homologous chromosomes:
Let’s assume that 3 monovalents do not occur.
 

Chromosome 1 Chromosome 2 Chromosome 3 Total pairing association
III III III 3III
II + I II + I II + I 3II + 3I
III III II + I 2III + II + I
III II + I III 2III + II + I
II + I III III 2III + II + I
II + I II + I III III + 2II + 2I
II + I III II + I III + 2II + 2I
III II + I II + I III + 2II + 2I

What can be concluded about meiosis of triploids?

1. Meiocytes display more than one possible pairing association.
2. Meiosis is abnormal because meiocytes to not display only bivalent pairing.
3. Meiosis is also variable because meiocytes display more than one type of pairing association.
4. The probability that a triploid’s gametes are unbalanced is high. Therefore, it is expected that triploids are rather infertile.

Chromosome segregation of a triploid of 2n = 9 = 3x

1. Bivalents segregate 1 / 1. Each pole will receive one chromosome – balanced segregation. 
2. Trivalents segregate 2 /1 or 1 /2. This is an unbalanced segregation pattern.

Examples of segregation possibilities of trivalents

 3. There are a few possibilities for the monovalents. The behaviour of the monovalents varies from species to species:
- Monovalent may show directed movement to one pole at anaphase I and will be included in the forming nucleus of meiosis I. At anaphase II this monovalent will most likely undergo chromatid segregation and be included in the forming nucleus of meiosis II. X chromosome in male grasshoppers (X0) displays this behaviour.

 Directed movement of a monovalent


  
- Monovalent lags at anaphase I. Inclusion in a forming nucleus depends on the position of the monovalent in the cell during cytokinesis and may or may not be included in the forming meiosis I nucleus. If it is included it will most likely undergo chromatid segregation at anaphase II together with all the other chromosomes.

Random inclusion of monovalent
 

- Monovalent undergoes chromatid segregation at anaphase I. At anaphase II this monovalent will most likely not be included in the forming nucleus. It may, however, display directed movement to a pole.

Monovalent undergoes chromatid segregation

 
- Often monovalents that undergo chromatid segregation lag for a while before segregation commences. The outcome of this is that the segregants do not arrive at the poles in time to be included in the forming nucleus, especially if the nucleus is small.