The detailed cytogenetical studies have been undertaken in 25 members of genus Citrus, for the first time to determine the chromosomal basis of speciation.
Inversion heterozygosity has been observed in C. penvisculata. This is the first record of inversion heterozygosity in genus Citrus.
Cytomixis is being reported for the first time in this genus.
Binucleate PMCs have been observed in some species, their significance in evolution of polyploid forms has been discussed.
C. limonia var. Italian round shows inversion bridges, while the other variety, I. oblong does not show any bridge and fragment configuration. This clearly shows the role of inversion in evolution of new forms in Citrus.
C. sinensis var. Malta shows occasional breakdown of first division spindle leading to formation of restitution nucleus. The double division of chromosomes have also been observed in Malta.
The high pollen sterility (86%) in Marsh grape fruit is genic because fairly regular bivalent formation has been observed.
C. Assamensis is a translocation heterozygote showing trivalents, quadrivalents and sexivalent at meiosis. Inversion bridges have also been observed in this case. This, presumably, is the first record of translocation heterozygosity in genus Citrus. A univalent bridge has been observed at anaphase I. The origin of this species has been discussed.
The somatic chromosomes in Citrus species vary between 3.5μ and 1μ. Karyotypes differ in different species.
The importance of mitotic anomalies, in the production of bud variants, has been emphasized.
The inversion heterozygosity has been observed in 8 out of 25 members of genus Citrus.
The univalent chromosomes occur in 17 out of 25 members.
The pollen sterility in Citrus species varies between 86% (Marsh grape fruit) and 3% (C. rugulosa). The structural hybridity and presence of univalent chromosomes appear to be the main cause of pollen sterility.
The evolution in genus Citrus has been discussed in detail. The polyploidy has not played any significant role in evolution. The structural changes, gene mutations and hybridization appear to have dominated the evolutionary history of genus Citrus. The breakdown of meiosis and its significance, with regard to the origin and evolution of nucellar embryony, has been emphasized.