Mol Gen Genet. 20: 246(6): 657-62. (Mar 20, 1995)
On the occurrence of somatic meiosis in embryogenic carrot cell cultures.
Giorgetti L, Vergara MR, Evangelista M, Lo Schiavo F, Terzi M, Nuti Ronchi V.
Istituto di Mutagenesi e Differenziamento CNR, Pisa, Italy.

Abstract
During the establishment of an embryogenic cell line from a carrot hypocotyl explant, processes closely resembling meiotic divisions are seen. A microdensitometric analysis revealed that the amount of cellular DNA diminished in the majority of cells to the haploid level. However, the diploid level was re-established in a matter of a few days. The genetic consequences of this segregation were studied by analyzing restriction fragment length polymorphisms (RFLP) and randomly amplified polymorphic DNAs (RAPD). The results showed that the great majority of embryos regenerated from segregants and that different segregants had different genetic constitutions.

Introduction
It is an established fact that somatic cell cultures show an excess of recessive genetic variants (Siminovitch 1976; Terzi and Sung 1986; Terzi 1974). This is true both in animal and plant somatic cells in vitro, where recessive mutants appear with frequencies far in excess of those expected on the basis of the square of the mutation frequency observed in haploid conditions. Abundant data exist to show that in somaclonal regenerants expression of recessive mutations is extraordinarily frequent (Evans 1989): in a recent study of somaclonal variation, 18 out of 19 mutated regenerants bred true for at least three generations (Gavazzi et al. 1987).

Various proposals have been put forward to explain this excess of recessives: suggested mechanisms range from small deletions or gene inactivation (Terzi 1974) to mitotic crossing over (Evans 1989), but the direct experimental evidence, obtained in attempts to quantify the importance of these various mechanisms, seems to favour a mechanism involving chromosome loss and reduplication (Campbell and Worton 1981).

To our knowledge these facts have never been related to meiosis, the occurrence of which in somatic cells was described long ago: chromosome segregation and reduction in somatic tissues — usually followed at once by a restoration of the diploid number — have been described in a number of cases (Nuti Ronchi et al. 1992a,b; see Huskins 1948 for older work). The phenomenon, however, as pointed out by Huskins (1948) is "very likely to be missed by the cytologist unless the organism has grossly heteromorphic pairs of chromosomes".

Segregation events leading to haploidy were recently shown to occur in embryogenic carrot cell lines (Nuti Ronchi et al. 1992a,b). In the present work we describe meiotic figures in somatic cells and compare them with those that appear during microsporogenesis. In addition randomly amplified potymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP) patterns of pure lines, their sexual hybrid, and regenerants obtained from the latter via somatic embryogenesis, show that, in regenerants, segregation is indeed the rule rather than the exception. Hence somatic meiosis seems to be associated with embryogenicity or, more precisely, with its pre-requisite, viz. developmental totipotency.


References

See also Risley: Roses have satellites, too (1959)
"The number of chromosomes in the cells of one plant are supposed to be constant, and almost always are so. The full compliment of 21 chromosomes of the triploid offspring of Skinner's Rambler x Yellow Pinocchio are shown in Figure 11. All 21 were not always present in the root-tip cells. Only 33% contained all 21, while the remainder had 20, 19, 18 and 17 chromosomes per cell. This rose should produce an occasional mutation or sport from a bud if this condition of variable chromsomes number exists in all the tissues of the plant.*"

*Or if plants are regenerated from root cuttings,.

Burdick: Tomato Haploids (1951) also observed some peculiar offspring raised from doubled haploids.