Polyploids and haploids

In humans, changes in chromosome number and structure have major impacts on health.  In plants – both wild and domesticated – variation in chromosome number is widespread and of evolutionary and economic importance. 

Euploidy:  Euploidy is the situation where an organism has exact multiples of a basic ("x") chromosome number.  Diploids and polyploids with exact multiples of an "x" number are all euploids.

Aneuploidy: Aneuploids have more or less chromosomes than an exact multiple of the x number, e.g. 2x + 1. A bit more on aneuploids : NEXT LEVEL

For example, barley in the sporophytic generation is 2n = 14 and n = 7 in the gametophytic generation.

Since 7 x 2 = 14, barley is a diploid (euploid). The "base number (x)" is 7. In the case of a diploid, "x" is the same as the "n" number.

Polyploidy is the situation where an organism has other than two basic sets of chromosomes.  The two types of polyploidy are autopolyploidy and allopolyploidy.

Ploidy levels in the Triticeae (an allopolyploid series) where "x" = 7. Note: Based on a genome formula alone, you cannot tell if an organism is allopolyploid or autopolyploid.

A note on polyploid nomenclature:  NEXT LEVEL

Autopolyploid: A cell or individual whose several chromosome sets, three or more, are all homologous. These sets arise within a species via a process of genome multiplication.

Autopolyploids can be fully fertile (potato (2n = 4x = 48), alfalfa (2n = 4x = 32). Even though there are multiple homologous chromosomes, there are even numbers of homologs, and pairs of homologs show bivalent pairing. Therefore meiosis is normal and balanced gametes are produced.

Newly synthesized autopolyploids are usually sterile due to meiotic irregularities.

 

Sterility in autotetraploids due to uneven numbers of homologous chromosomes can be useful for producing seedless fruits.

 

Seedless (3x) watermelons are developed from 4x X 2x crosses

 

 

 

Considerations with autopolyploids:

Genetics and breeding: Can be quite complex, since potentially every allele at each locus can be different – e.g. an autotetraploid could have up to four different alleles at a locus. 

 

 

Sources: Autopolyploids can arise spontaneously from a somatic doubling of chromosome number and/or the formation of unreduced gametes, or they can be induced. Doubling chromosome numbers with colchicine: NEXT LEVEL.

 

 

• Allopolyploid: A cell or individual with genetically different chromosome sets derived from two or more species.

 

 

Allopolyploids arise through interspecific hybridization and spontaneous chromosome doubling. They can also be created by crossing two different species and doubling the chromosome number of the F₁ .

In general, allopolyploids behave like diploids due to bivalent pairing – each of the genomes behaves autonomously, although the multiple homoeologous genomes may be collinear.

Pairing is under genetic control - mutations can lead to pairing of homoeologous chromosomes and sterility.

 

 Example: Bread wheat: a natural allopolyploid

Wild einkorn        X             Unknown 2n=2x=14; AA                   2n=2x=14; BB ¯ “F1” Hybrid = AB = sterile spontaneous chromosome doubling ¯ Wild emmer wheat 2 n = 4x = 28; AABB ¯ Domestication ¯               Cultivated emmer wheat     X   Wild diploid (Aegilops squarrosa; T. tauschii) 2 n = 4x = 28; AABB                     2n = 2x= 14; DD ¯ F1 ABD sterile spontaneous chromosome doubling ¯ Common wheat 2n = 6x = 42; AABBDD

 

 

 

Example: Triticale: a man-made allopolyploid.

Rye            X          Durum wheat          2n=2x = 14; RR              2n=4x=28 = AABB ¯ F1  = ABR = sterile colchicine chromosome doubling ¯ Hexaploid triticale AABBRR = 2n=6x=42

 

Triploidy leads to seedless fruits in allopolyploids:

The banana is 2n = 3x = 33. More on bananas and genetic vulnerability

The parting shot on polyploids. "With the advent of genome sequencing and the availability of extensive EST data and high-density, molecular marker-based maps, it became clear that all plant genomes harbor evidence of cyclical, recurrent episodes of genome doubling." (Udall and Wendel, 2006)

Haploids: A haploid has a single basic set of chromosomes: e.g. maize is n=10; barley is n=7; and hexaploid wheat is n = 21. 

A haploid plant will grow and look quite like a normal but, as it has only the "n" number of chromosomes, it is sterile. Sterility is due to the fact that there is only one homologous chromosome per nucleus. With no counterpart to pair with, meiosis will lead to the production of imbalanced gametes.

In the context of plant breeding and genetics, the utility comes with doubled haploids.

The reason is that when the chromosome complement of a haploid is doubled, you create an “instant homozygote”. If these instant homozygotes are produced from F₂ gametes (i.e. pollen or eggs of F₁ plants), then one can sample the range of segregation and independent assortment possible within the cross. Of course, population size is critical in terms of recovering as many unique configurations of alleles at multiple loci as possible. The key point with a doubled haploid population is that there are no heterozygotes. 

Doubled haploids can be produced by a number of mechanisms.

The two most common mechanisms can be classified as androgenetic (i.e. male-based systems) or gynogenetic (i.e. female-based systems).

 

Within the androgenetic systems, there are anther and microspore culture.

 

Within the gynogenetic systems, there are chromosome elimination and ovule culture.