Cytological and Morphological Studies in the Genus Sorghum

Abstract

The morphological characters of nine species and six F₁ hybrids were compared. It was found that with respect to characters such as tillering, panicle branching, floret shape, colour of the midrib, stigma and grains, the hybrids were intermediate but for characters such as plant height, leaf length and number of nodes, the hybrids were usually heterotic. The awned condition was found to be recessive to awnlessness and the non-staminate condition of pedicellate florets to staminate condition.
Critical analysis of chromosomes at pachytene and later stages of microsporogenesis in the seven Eu-sorghum species and six F₁ hybrids were made. Pachytene chromosomes are depicted on the basis of total length, relative length, arm-ratio, amount and distribution of heterochromatin and number and position of chromomeres. The characteristics of each of the ten chromosomes in the haploid complement are described in ten different classes bringing out the similarities and differences among these seven species. Studies on the pairing properties of the differentially stained regions show that synapsis starts from the proximal to the distal and separation of the split chromosomes starts from the distal to the proximal.
Pachytene analysis in the four F₁ hybrids revealed the presence of some minute, though cytologically detectable, structural differences such as terminal as well as interstitial non-paired regions, small duplication and small terminl deletions and differential segments between the parental species. Pachytene pairing was found to be complete and apparently normal in the remaining two hybrids. Inspite of the existence of these meiotic irregularities in some hybrids, high percentage of stainable pollen and good seed setting were recorded in all the hybrids under consideration.
The distribution of chiasmata at various stages was studied in both species and species hybrids and the average chiasma frequency did not show any significant deviation from those in the parental species. It is suggested that the existing chromosomal differences are quite small and do not lead to abnormal behaviour of the hybrids either in respect of morphological characters or in the meiotic process in the F₁ generation.
Cytogenetical mechanisms underlying species differentiation in the genus are discussed.