Cytogenetics and evolution of cassava (Manihot esculenta Crantz)

Nagib M.A. Nassar

doi:10.1590/s1415-47572000000400046

Abstract

All Manihot species so far examined, including cassava (Manihot esculenta), have 2n = 36. Interspecific hybrids between cassava and its wild relatives show fair regular meiosis, and backcrossed generations exhibit high fertility. Electrophoresis shows affinity between species of different sections, as well as between some of them and cassava itself. Polyploidy has apparently contributed to the rapid speciation of this genus, while apomixis has offered a means of perpetuating new hybrid types adapted to different environments. It is assumed that cassava originated by hybridization between two wild Manihot species followed by vegetative reproduction of the hybrid.

Highlights

All Manihot species so far examined, including cassava (Manihot esculenta), have 2n = 36
Harlan (1961) has shown that more than one diversity center may be formed for a given crop through introgression, which may explain why, in many cases, we find centers of diversity for a given crop very far from areas of diversity of the crop wild relatives
All wild Manihot species examined cytogenetically have a high chromosome number of 2n = 36 (Nassar, 1978a; Table II), but in spite of this the species behaves meiotically as diploids and it is believed that allopolyploidization caused the emergence of the whole group and was responsible for both its rapid speciation and the weak interspecific barriers which lead to interspecific hybridization

Summary

Introduction

Manihot diversity micro-centers exist in central Brazil, where large numbers of species are concentrated in small areas less than 50 km in diameter (Nassar, 1978b,c; 1986), allowing frequent hybridization between species in topographically heterogeneous habitats which help to isolate fragmented gene pools leading to speciation. All wild Manihot species examined cytogenetically have a high chromosome number of 2n = 36 (Nassar, 1978a; Table II), but in spite of this the species behaves meiotically as diploids and it is believed that allopolyploidization caused the emergence of the whole group and was responsible for both its rapid speciation and the weak interspecific barriers which lead to interspecific hybridization.

Results

Conclusion