H

Eranad Chernad, Thodupuzha

Source: Ratnambal (1979).

Source: Ratnambal (1979).

macrostachyum the total chromatin length of the haploid complement is 29.6 ^m. The absolute length of individual chromosomes varies from 3.5 to 1.9 ^m. Chromosomes 1, 2, 4, 5, and 9 have submedian centromeres; 3, 6, 10, and 11 have median centromeres; and 7 has terminal centromeres. The second chromosome had a satellite on its longer arm. Z. cassumunnar had a total chromatin length of 24.7 ^m; the individual chromosomes length varied from 2.9 to 1.6 fxm. The karyotype is characterized by one subterminal, two submedian (one of which is satellited), and eight median chromosomes. There were three long, three medium, and five short chromosomes. In all three species the type of asymmetry is reported as being IA.

Das et al. (1998) carried out karyotype analysis and 4C DNA estimation in eight ginger cultivars. They recognized five types of karyotypes occurring in these cultivars.

Type A. Large- to medium-sized chromosome with primary and secondary constrictions nearly submedian in position, respectively

Type B. Large- to medium-sized chromosome with two constrictions, one in the submedian position and other in the subterminal position

Type C. Small-sized chromosome with nearly median to median primary constriction with satellite bodies on the long arm

Type D. Medium- to small-sized chromosome with nearly submedian primary constriction

Type E. Medium-sized chromosomes with nearly submedian primary constriction

All the types of karyotypes are found in the cvs. Bhitarkata local, Himachal Pradesh, and Tura. The A type was present in all the cultivars except in cvs. Raipur local and Wynad. The C type chromosome was common in all the cultivars except in cvs. Maran, Nadia, S-557, and Tura. D and E types were found in all the cultivars. The total chromosome length ranged from 64.80 fxm in cv. S.557 to 98.12 fxm in cv. Wynad. Total chromosome volume was from 84.35 fxm3 in S.557 to 1126.36 fxm3 in Wynad.

The 4C DNA varied significantly in different cultivars of ginger; from 16.234 pico-gram (pg) in cv. S.537 to 22.934 pg in cv.Wynad. The average chromosome length and volume ranged from 2.94 to 4.46 and 3.83 to 5.74 ^m3, respectively. The nuclear DNA content was directly proportional to the total chromosome volume, which in turn was positively correlated with the chromosome length. The variability in DNA amount has been attributed to loss or addition of highly repetitive DNA sequence rather than the adenine-thymine (AT) or guanine-cytosine (GC) rich sequences in a genome, which reached a certain level and became stabilized during microevolution and gradual selection (Das et al., 1998).

Meiosis

Ratnambal (1979) and Ratnambal and Nair (1981) studied the process of meiosis in 25 cultivars of ginger. These cultivars exhibited much intercultivar variability in meiotic behavior. Cultivars like Karakkal formed only bivalents, whereas in cv. Taiwan two hexavalents, one quadrivalent, and three bivalents were present. Univalents were very common and much variability was noticed in respect of their number (Figure 2.12a, b). The presence of multivalent and chromatin bridges was found to be a common feature in most cultivars studied by Ratnambal (1979). The presence of multivalents in a diploid species indicates structural hybridity involving segmental interchanges, and four to six chromosomes are involved in the translocations as evidenced by quadrivalents and hexavalents. This structural hybridity might be contributing to the sterility in ginger.

Ratnambal (1979) also reported two to six univalents in various cultivars; the lowest was in the cv. Mananthody and the highest in cv. Karakkal. The number of univalents observed at metaphase 1 was more than that in diakinesis, and this has been attributed to the precocious separation of one or two bivalents. Most of these univalents end up in the formation of micronuclei and are lost subsequently. This leads to the production of gametes with deficiency and is likely to lead to sterility. A high percentage of abnormalities has been observed during the first and second divisions, as well as in the tetrad stage. The bridges noticed were presumed to be due to inversion heterozygosity or from chromosomal breakage and reunion in the early stage of meiosis. Unequal breakage of bridges at anaphase might be leading to the production of gametes with duplications and deficiencies (Ratnambal, 1979).

Structural chromosomal aberrations occurred at all stages of microsporogenesis in ginger. The predominant aberrations were laggards, bridges, and fragments at anaphase I; laggards, bridges, and fragments, irregular chromosome separation, and irregular cytokinesis at anaphase II; and micronuclei and supernumerary spores at the quartet stage (Table 2.6). Ratnambal (1979) had shown a positive linear regression between pollen sterility and chromosomal aberrations at anaphase II and aberrant quartets. Structural chromosomal aberrations have been attributed as the cause of sterility in ginger. But how such a diploid species as ginger came to acquire a complicated meiotic system that led to chromosomal sterility is not well understood. A hybrid origin followed by continuous vegetative propagation can be one reason for the abnormal chromosomal behavior (Ratnambal, 1979). Beltram and Kam (1984) studied meiotic features of 33 species in Zingiberaceae, including nine species of Zingiber. They observed various abnormalities such as aneuploidy, polyploidy, and B chromosomes. They also confirmed the diploid nature of the Malysian Zingiber (x = 11) and the pentaploid nature of the Japanese ginger, Z. mioga.

Das et al. (1998) studied meiosis and sterility in four cultivars (Bhaisey, Ernad Chernad, Gorubathany, and Thuria local) and reported a 30.35 to 40.5% meiotic index in them. Pollen mother cells showed incomplete homologous pairing at metaphase 1 and spindle

Figure 2.12 (a) Meiosis (diakinesis) showing multivalents. (b) Meiotic metaphase showing multivalents and univalents.

abnormalities (e.g., late separation, laggards, sticky bridges) at anaphase 1, leading to high pollen sterility. Das et al. (1999) felt that the sterility might be due to nonhomology of bivalents, with irregular separation of genomic complements leading to sterile gamete formation. The absence of germination pores on the pollen grains has also been indicated as an impediment to seed set.

Table 2.6 Chromosomal abnormalities and pollen sterility in ginger cultivars

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