Cytological evidence on the origin of sweet potato

Abstract

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.

Disentangling the Origins of Cultivated Sweet Potato (Ipomoea batatas (L.) Lam.)

“…The autopolyploidization scenario assumes I. trifida to represent an autopolyploid complex, with different ploidy levels (from diploid to hexaploid) from which cultivated I. batatas derived. However, cytological and marker-based studies suggested that the I. batatas hexaploid genome may be composed of two closely related genomes and a third one from a more distant relative [19], [44]. Austin [25] postulated that the wild ancestor of I. batatas was a hybrid between I. trifida and I. triloba (allopolyploid scenario).…”

Phenotypic diversity of plant morphological and root descriptor traits within a sweet potato, Ipomoea batatas (L.) Lam., germplasm collection from Tanzania

“…The sweet potato crop has a wide adaptability on marginal land and has the potential to alleviate malnutrition and enhance food security in the developing world (Diaz et al 1996). Sweet potato is hexaploid (2n = 6x = 90) and is highly selfincompatible (Magoon et al 1970;Ozias-Akins and Jarret 1994). More than 8,000 accessions, cultivars and breeding lines of sweet potato and nearly 26,000 accessions of other Ipomoea species are maintained at 83 gene banks in the world (Kuo 1991;Rao et al 1994).…”

Root and Tuber Crops

“…Morphological analysis of the related species indicates that I. trifida is the closest wild relative to sweet potato but I. tabascana is also morphologically very close (Austin, 1977(Austin, , 1987. The most compelling evidence of an allopolyploid origin is cytological data reported by Magoon et al (1970). G. Don and wild Mexican tetraploids are putative ancestors of the cultivated sweet potato.…”