KARYOLOGICAL STUDIES IN AMORPHOPHALLUS CAMPANULATUS

Karyomorphological studies of Amorphophallus campanulatus Blume (Araceae) were pursued at pachytene and at root tip and pollen mitotic metaphases. The haploid complement (n = 14) at root and pollen mitoses could be categorized under six types including two nucleolar chromosomes. The karyotypes at these two stages were in agreement except for the relative lengths of one of the nucleolar chromosomes. The chromosomes at prophase of pollen mitosis and pachytene were characterized by differential staining. The two nucleolar chromosomes at pollen mitosis and pachytene were similar in general morphology and length relationships. Adopting suitable morphological criteria, the haploid chromosomal complement is identified. Based on this data and other considerations, a polyploid origin for this species was suggested and the urgent need for a comprehensive cytological survey of this genus is stressed.

Cytological evidence on the origin of sweet potato

The results of intensive meiotic studies, particularly of the karyology and chromosomal homology at the pachytene stage, in the sweet potato (Ipomoea batatas L.), which is a hexaploid (2 n = 90), have thrown considerable light on its origin and genome relationships. Using suitable criteria, such as relative length of chromosomes, centromere position, chromomere pattern, absence of light staining segments in one of the arms, presence of telochromomere etc., 40 of the 45 haploid chromosome complement at pachytene were identified and assigned to 19 chromosomal types. Among these types, eight were present singly; in six of the types, chromosomes were present in duplicate, and in two types, in triplicate. The occurrence of higher multivalent chromosomal associations such as hexavalents and pentavalents, in addition to the quadrivalents already reported, was recorded for the first time at the pachytene and metaphase I stages. The hexavalents at pachytene were resolved into three distinct types based on the morphology of the participating chromosomes. A maximum number of nine quadrivalents at the metaphase I stage and four in the incompletely analyzed pachytene nuclei were recorded. The constituent chromosomes of three of the quadrivalents at pachytene stage were identified. From these observations, it is suggested that (i) the three parental genomes are partly homologous (ii) two of the genomes show closer homology to one another than to the third and (iii) the three genomes differ with respect to one or more of the eight chromosomal types occurring singly. The available information rules out an autopolyploid origin for sweet potato and suggests that the parental genomes are from closely related taxa. The advantages are emphasized of pursuing similar studies in other American Ipomoea species to unravel their relationship with the sweet potato. Among other meiotic irregularities, a translocated chromosome and a chromosome carrying inversion were detected at the pachytene stage and the possible role they may play in varietal differentiation is discussed.

The pachytene chromosomes of Ipomoea crassicaulis

The detailed morphology of the pachytene chromosomes and microsporogenesis have been studied in a diploid (2 n=30) American species, Ipomoea crassicaulis (Bth) B. L. Robinson. Idiogram of the pachytene chromosomes is presented and taking advantage of the extreme precision that pachytene analysis can lend, karyological characteristics of the haploid complement have been worked out in detail and individual chromosomes are identified. The course of meiosis was normal and over ninety five percent of pollen were found stainable. The urgent need for extending similar studies to other taxa in this economically important genus for unravelling phyletic relationships has been stressed.