Chromosome pairing in interspecific hybrids between Coix aquatica (2n=10) and C. lacryma-jobi (2n=20)

Reciprocal hybrids between C. aquatica and C. lacryma-jobi, obtained naturally and by deliberate crossing, showed 2n=15 chromosomes. Chromosome pairing was studied in these hybrids at diakinesis and metaphase I. Although at pachytene 6 of the chromosomes in C. lacryma-jobi are longer than the shortest chromosome in C. aquatica (J. Venkateswarlu, R. S. K. Chaganti, and P. N. Rao, 1976, Bot. Mus. Leafl. Harvard Univ. 14:205-224), in the hybrids, from diakinesis onwards, the 5 aquatica chromosomes appear distinctly longer than all the chromosomes of lacryma-jobi, due to differential levels of heteropycnosity and condensation of chromosomes in the two species. This distinction of chromosomes belonging to the two genomes is substantiated from chromosome behavior in triploid hybrids (3n=25) between induced tetraploid lacryma-jobi (4n=40) and C. aquatica (2n=10) in which the 20 small chromosomes of lacryma-jobi formed 10 bivalents and the 5 large chromosomes of aquatica mostly remained as univalents.

This distinctive feature of the chromosomes of the two species, when present in the same nucleus, facilitated the study of intergenomic and intragenomic pairing in the hybrids. The highest associations encountered were two associations of 3 chromosomes each (mostly comprising one aquatica and two lacryma-jobi chromosomes) with a mean of 0.328 per cell. Heteromorphic bivalents involving aquatica and lacryma-jobi chromosomes occurred with an average of 2.121 and a range of 0 to 5 per cell. These were mostly rods and occasionally rings. Intragenomic bivalents among aquatica chromosomes were predominantly rods, and occurred with an average of 0.155 per cell. The aquatica univalents ranged between 0 and 5 with a mean of 2.224 per cell. They occasionally showed inter-arm pairing. Pairing amongst lacryma-jobi chromosomes was quite rare, occurring with a frequency of not more than one bivalent per cell with a mean of 0.086. The lacryma-jobi univalents ranged between 2 and 10 with an average of 7.086 per cell. Further divisions of meiosis were very irregular, leading to near complete pollen sterility and meagre seed set even under open pollinated conditions.

The meiotic behavior in hybrids provides some understanding of the genomic constitution of the two species. The occurrence of 5 heteromorphic bivalents suggests that in C. lacryma-jobi there are 2 genomes of 5 chromosomes each and that one of them is apparently derived from C. aquatica. The formation of higher associations of 3 chromosomes and occasional intragenomic pairing in lacryma-jobi chromosomes suggests that the second genome of 5 chromosomes is related to the first one. The occurrence of inter-arm pairing in univalent chromosomes and intragenomic bivalents in aquatica complement reveal the existence of both intra and interchromosomal duplications in the aquatica genome.

Panuganti N. Rao


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