Wednesday, July 17, 2024
General Agriculture

Tetraploidy and Higher Polyploidy

Tetraploids are the second group of polyploids after triploids. As the name suggests, tetraploids have 4x chromosome complement. This means that each chromosome type occurs in quadruplicate. Unlike triploids that are usually autotriploids, autotetraploids and allotetraploids occur frequently in plants.

Autotetraploids

Autotetraploids have 4x chromosome complement with genome formula AAAA.

Production of Autotetraploids

Production involves 2 mechanisms:

Genetic non-reduction and

Somatic doubling

Genetic Non-reduction (Refer to production of autotriploids)

Somatic Doubling:

Tetraploidy and Higher Polyploidy

Chromosome doubling can be spontaneous or induced. Induction is through colchicines treatment, a poisonous alkaloid drug that binds to tubulin, the major protein component of the spindle thereby preventing formation of the spindle apparatus.

In cells without spindle apparatus, the sister chromatids do not separate after the centromere

splits, so the chromosome no doubles. Since there is no spindle, metaphase chromosomes (colchicines metaphase or c- metaphase) remain scattered in the cytoplasm.

Continuous colchicines treatment may cause reduplication. For instance, onion cells (2n = 16) bathed with colchicines for 4 days may contain 1,000 chromosome per nucleus.

Fertility in Autotetraploid

Because 4 is an even number, autotetraploids can have a higher meiosis. The crucial factor is how the four homologous chromosomes pair and segregate.

There are several possibilities which include two bivalents, one quadrivalent and a univalent-trivalent association.

In tetraploidds, the two-bivalent and the quadrivalent pairing modes tend to be most regular, even here there is no guarantee for a 2:2 segregation.

If all chromosome sets segregate 2:2 as they do in some species, then the gametes will be functional and genetic analysis can be made.

Genetics of a Fertile Autotetraploid

Determination of Genotypes

In an autotetraploid, a gene is represented 4 times at a locus. Thus we can have the following allelic constitutions with reference to the dominant allele ‘A’ at locus A/a.

AAAA – quadriplex

AAAa – triplex

AAaa – duplex

Aaaa – simplex

aaaa – nulliplex

Gamete production is a duplex – Aaaa.

Note: We have further concern whether the locus in question is tightly linked or not to the centromere since the two situations give different results.

Case 1: Locus is not linked to the centromere.

In this case crossing-over must be considered. This forces us to think in terms of chromatids.

The packaging of genes two at a time into games is very much like grabbing two balls at random from a bag of eight balls: 4 of one kind, 4 of another. The probability of picking two b genes = probability of bb gamete.

= 4/8 (the first one) x 3/7 (the 2nd one) = 3/14

... b/b/b/b = (3/14)2 = 9/196

Allotetraploids

1. Classical Allotetraploid–Raphinobrassica

The “classic allotetraploid” was synthesized by G. Karpechenko in 1928. He wanted to make a fertile hybrid that would have the leaves of the cabbage (Brassica) and the roots of the radish (Raphanus).

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Each of these species has 18 chromosomes, and they are related closely enough to allow intercrossing. A viable hybrid progeny individual was produced from seed.

However, this hybrid was functionally sterile because the nine chromosomes from the cabbage percent were different enough from the radish chromosomes that pairs did not synapse and disjoin normally:

Tetraploidy and Higher Polyploidy
Tetraploidy and Higher Polyploidy

n1 + n2= 18

Sterilehybrid

However, one day a few seeds were in fact produced by this (almost) sterile hybrid. On planting, these seeds produced fertile individuals with 36 chromosomes. All these individuals were allopolyploids.

They had apparently been derived from spontaneous, accidental chromosome doubling to 2n1 + 2n2in the sterile hybrid, presumably in tissue that eventually became germinal and underwent meiosis. Thus, in 2n1 + 2n2tissue, there is a pairing partner for each chromosome and balanced gametes of the type n1 + n2are produced.

These gametes fuse to give 2n1 +2n2allopolyploid progeny, which also are fertile. This kind of allopolyploid is sometimes called an amphidiploids, which means “doubled diploid” (Fig.). (Unfortunately for Ka rpechenko, his amphidiploids had the roots of a cabbage and the leaves of a radish).

When the allopolyploid was crossed with either parental species, sterile offspring resulted. The offspring of the cross with radish were 2n1+n2, constituted from an n1 + n2gamete from the allopolyploid and an n1 gamete from the radish.

The n2chromosomes had no pairing partners, so sterility resulted. Consequently, Karpechenko had effectively created a new species, with no possibility of gene exchange with its parents. He called his new species Raphanobrassica.

Production of Allotetraploid

It is produced by hybridization of 2 different species to yield an infertile hybrid in F1. F1 must be able to propagate vegetatively before chromosome doubling to produce a vigorous fertile hybrid.

1. Production of Allotetraploid Triticale by Hybridization

Today, allopolyploids are routinely synthesized in plant breeding. Instead of waiting for spontaneous doubling to occur in the sterile hybrid, the plant breeder adds colchicines to induce doubling.

The goal of the breeder is to combine some of the useful features of both parental species into one type. This kind of endeavor is very unpredictable, as Karpechenko learned. In fact, only one synthetic amphidiploids has ever been widely used.

This amphidiploids is Triticale, an amphiphidiploid between wheat (Triticum, 2n = 6x = 42) and rye (Secale, 2n = 2x = 14). Triticale combines the high yields of wheat with the ruggedness of rye. Figure shows the procedure for synthesizing Triticale.

In conclusion, Autotetraploids and allotetraploids can occur spontaneous or artificially in plant breeding. Tetraploids are usually produ ced in plant breeding because of their commercial and other economic advantages over their diploid counterparts.

There are two major types of tetraploids namely autotetraploids and allotetraploids. Tetraploids have considerably high fertility because the ploidy is even (i.e. 4x). Several mechanisms ensure that normal bivalents are formed as is the case with normal diploids.

Thus, an odd number of chromosome sets makes an organism sterile because there is not a partner for each chromosome at meiosis, whereas even number of sets can produce standard segregation ratios to cause fertility.

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Benadine Nonye is an agricultural consultant and a writer with several years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education - PhD Student in Agricultural Economics and Environmental Policy... Visit My Websites On: 1. Agric4Profits.com - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. WealthinWastes.com - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and WealthInWastes TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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