Dear Sarbesh,

The following text taken from a review entitled "An Introduction to Genetic Analysis. 7th edition. " covers the answer to your question.

http://www.ncbi.nlm.nih.gov/books/NBK22089/

Polyploids

In the realm of polyploids, we must distinguish between autopolyploids, which are composed of multiple sets from within one species, and allopolyploids, which are composed of sets from different species. Allopolyploids form only between closely related species; however, the different chromosome sets are homeologous (only partly homologous)—not fully homologous, as they are in autopolyploids.

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Triploids

Triploids are usually autopolyploids. They arise spontaneously in nature or are constructed by geneticists from the crossof a 4x (tetraploid) and a 2x (diploid). The 2x and the x gametes unite to form a 3x triploid.

Triploids are characteristically sterile. The problem, like that of monoploids, lies in pairing at meiosis. Synapsis, or true pairing, can take place only between two chromosomes, but one chromosome can pair with one partner along part of its length and with another along the remainder, which gives rise to an association of three chromosomes. Paired chromosomes of the type found in diploids are called bivalents. Associations of three chromosomes are calledtrivalents, and unpaired chromosomes are called univalents. Hence in triploids there are two pairing possibilities, resulting either in a trivalent or in a bivalent plus a univalent. Paired centromeres segregate to opposite poles, but unpaired centromeres pass to either pole randomly. We see in Figure 18-4 that the net result of both the pairing possibilities is an uneven segregation, with two chromosomes going in one direction and one in the other. This happens for every chromosome threesome.

Figure 18-4

Two possibilities for the pairing of three homologous chromosomes before the first meiotic division in a triploid. Notice that the outcome will be the same in both cases: one resulting cell will receive two chromosomes and the other will receive just (more...)

If all the single chromosomes pass to the same pole and simultaneously the other two chromosomes pass to the opposite pole, then the gametes formed will be haploid and diploid. The probability of this type of meiosis will be (1/2)x−1, and this proportion is likely to be low. All other possibilities will give gametes with chromosome numbers intermediate between the haploid and diploid number; such genomes are aneuploid—“not euploid.” It is likely that these aneuploid gametes will not lead to viable progeny; in fact, this category is responsible for the almost complete lack of fertility of triploids. The problem is one of genome imbalance, a phenomenon that we shall encounter repeatedly in this chapter. For most organisms, the euploid chromosome set is a finely tuned set of genes in relative proportions that seem to be functionally significant. Multiples of this set are tolerated because there is no change in the relative proportions of genes. However, the addition of one or more extra chromosomes is nearly always deleterious because the proportions of genes in those extra chromosomes are altered. Although the action of some genes can be regulated to compensate for extragene “dosage,” the overall effect of the extra genetic material seems too great to be overcome by gene regulation. The deleterious effect can be expressed at the level of gametes, making them nonfunctional, or at the level of the zygote, resulting in lethality, sterility, or lowered fitness.

In triploids, it is possible that some haploid or diploid gametes will form, and some may unite to form a euploid zygote, but the likelihood of this possibility is inherently low. Consider bananas. The bananas that are widely available commercially are triploids with 11 chromosomes in each set (3x = 33). The probability of a meiosis in which all univalents pass to the same pole is (1/2)x−1, or (1/2)10 = 1/1024, so bananas are effectively sterile. The most obvious expression of the sterility of bananas is that there are no seeds in the fruit that we eat. Another example of the commercial exploitation of triploidy in plants is the production of triploid watermelons. For the same reasons that bananas are seedless, triploid watermelons are seedless, a phenotype favored by some for its convenience.

MESSAGE

Some types of chromosome mutations are themselves aneuploid; other types produce aneuploid gametes or zygotes. Aneuploidy is nearly always deleterious because of genetic imbalance—the ratio of genes is different from that in euploids and interferes with the normal operation of the genome.

Hoping this will be helpful,

Rafik

In animals as the sex is generally determined by chromosomes, triploidy and pentaploidy causecauses severe imbalance and chromosomal anamolies, which are usually seen in the form of multivalents or univalents at M1.

Triploid and pentaploid deciduous azaleas are sometimes fertile where the pentaploids are fertile a higher percentage of time than the triploids.

In southeast North America contact zones of diploid and tetraploid deciduous azaleas species, triploids are often found. In some cases, viable open pollinated seed is found on triploids in these contact zones.

Moreover, in such contact zones, there is clear indications of bidirectional introgression between the diploid and tetraploid species. 

Such bidirectional introgression has been found in birches in Iceland. Here is a line from an abstract on Iceland birches.

Hybridization between the diploid Betula nana and the tetraploid Betula pubescens is widespread in Iceland. The overlapping morphological variation indicates bidirectional introgression between the two species via triploid hybrids.

The triploid bridge is likely in play for North American deciduous azaleas also.

The meiosis of triploids and pentaploids is normally uneven distributed splitting centered near 1.5x and 2.5x respectively where unreduced gametes are sometimes produced by both. The gametes produced by triploids and pentaploids are often not viable but gametes close to 1x, 2x, 3x, 4x can be.

The offspring of crosses of 3x X 4x deciduous azaleas are triploids, tetraploids, and pentaploids plus aneuploids between triploid and tetraploid.

Sarbesh, although the question of why are triploids and pentaploids often sterile is interesting, more interesting is how important are triploids and pentaploids to the evolution of flowering plants especially in those instances where the triploids successfully interact sexually with the existing diploid species.