In this article we will discuss about:- 1. Origin and Occurrence of Triploids 2. Chromosome Pairing and Chiasma Formation in Auto-Triploids 3. Fertility of Auto-Triploids 4. Pneotypic Effects of Auto-Triploidy.
Origin and Occurrence of Triploids:
(i) Triploids originate from hybridization between auto-tetraploids (4x) and diploids (2x). The autotetraploid produces diploid (2a) gametes which when fertilized by a normal monoploid (x) gamete produce triploid (3x) progeny.
(ii) Triploids have been produced in barley by crossing autotetraploid with mixoploid plants (Plate III : F, G). In such crosses, Prasad et al. in 1983 obtained triploids and tetraploids with a frequency of 32.6% and 3.6%, respectively.
(iii) Occasionally triploids occur among the progeny of diploids. They originate when an unreduced gamete (2x) becomes functional, and is fertilized by a normal (x) pollen.
(iv) Triploids have been found in the progeny of plants homozygous for “triploid-inducer” recessive gene, detected in barley by Ahokas in 1977. Plants homozygous for the triploid-inducer gene produce diploid and triploid seeds in about equal proportion. This gene suppresses the second meiotic division in about half of the megaspore mother cells so that nearly 50% of embryo sacs are diploid.
(v) Triploids may be obtained in the progeny of barley infected with stripe mosaic virus. In human, triploids originate in one of the following three ways:
(a) Fertilization of a diploid egg by a haploid sperm.
(b) Fertilization of a haploid egg by a diploid sperm.
(c) Fertilization of a haploid egg by two haploid sperms (dispermy).
Chromosome Pairing and Chiasma Formation in Auto-Triploids:
Triploids may be viewed as multiple trisomies, i.e., three sets of homologous chromosomes are present for all the chromosomes of the genome. The three homologues of a single chromosome can form a trivalent. Meiotic pairing occurs in a two-by-two fashion at any given point.
However, double synaptinemal complexes over short segments are also observed in some organisms, e.g., triploid lily and triploid chicken. When all the three homologues of a chromosome pair, trivalent formation is dependent on the occurrence of chiasma (ta) in the paired regions (Fig. 16.11).
The position where one chromosome changes its pairing association from one pairing partner to the other, is called the point of partner exchange. In general, chiasma frequency declines around the point of partner exchange. If two chromosomes pair, while the third remains unpaired a bivalent plus one univalent are formed for each such chromosome.
At Al, one or two of the three homologous chromosomes may go to the some pole, the remaining chromosome(s) moving to the opposite pole. Sometimes, the chromosomes may lag at the metaphase plate (Plate III : J, K). In triploid Datura stramonium (3x = 36), Satina and Blakeslee in 1937, found a strong tendency for the chromosomes to pass together in groups to one pole, rather than at random.
Frequencies of 12-24, 13-23, 14-22 and 15-21 segregations were greater than expected. They also found that the number of nuclei with lagging chromosomes was rather high in female (over 50%) but quite low in male (3.5%) spore mother cells. In barley triploids, lagging chromosomes were observed in 66.6% of the PMC’s.
Fertility of Auto-Triploids:
Triploids are highly sterile mainly due to chromosomal dis-balance in the spores; the degree of sterility varies in different species. In certain species, such as, Spinacea oleracea, triploids show normal seed set. In contrast, seed set in barley was 9.9% in eutriploids (3x= 21), 7.3% in hypo-triploids (3x – 1 = 20) and 6.8% in hyper-triploids (3x + 1 = 22).
The progeny of triploids consist of diploids and different kinds of aneuploids, e.g., primary trisomies (2x + 1), double trisomies (2x + 1 + 1), triple trisomies (2x +1 + 1 + 1), tetrasomics 2x + 2). pentasomics (2x + 3) etc. On selfing of triploids tetraploids, are also obtained but with low frequency.
Pneotypic Effects of Auto-Triploidy:
Triploid plants are more vigorous than diploids and are seedless. Thus they offer unique commercial advantages.Triploid water melons, first produced by Kihara in Japan from 4x x 2x crosses are seedless, hence they are easier to eat than the diploid seed-filled fruits. Bananas are natural triploids (3x = 33) and fruits are used as human food. Triploid sugar beet yields more sugar as compared to diploids and tetraploids.
In human, triploid embryos are mostly aborted, but some babies have been born and survived for a few days. Triploid babies which survive for more than few days are not true triploids but they are diploid-triploid mosaics. Triploid babies have smaller skull, show syndactyly, malformed ears and eyes, cleft palate and mental retardation.