of genomes (see Appendix 2) without demonstrating a clear evolutionary
connection between the number of chromosomes and the specificity of
organisms or the complexity of their organization.
From the data of Appendix 2 it is seen that the haploid chromosome
set (1n) varies in large proportions in the various types of organisms from 1
to 520. The apprehensions of some authors regarding the virtual nonexistence
of a minimum chromosome set of only one chromosome (1n = 1)
in eukaryotes proved to be in vain. Crosland and Grozier (1986)
demonstrated the availability of such a genome in Australian ant Myrmecia
pilosula (Fig. 2–19). According to these authors a report of one
chromosome in the nematode Parascaris equorum univalens was
presented by T. Boveri even in 1887, but has remained unnoticed.
image
Figure 2–19. Chromosomes from prepupal cerebral ganglia: (A) Worker ant prometaphase chromosomes. Identical C-banding provides evidence for homology of the two chromosomes; (B) Male prometaphase chromosome. Chromosomes consistently display a large centromeric C-band on the short arm and a smaller centromeric C-band on the long arm. Most of the short arm C-band is not immediately adjacent to the centromere, though a very small portion of the short-arm C-band is centromeric. Arrows indicate position of centromere (After Crosland and Grozier, 1986).
Genomes consisting of two chromosomes (1n = 2) are known in the
parasitic round worms (Ascaris), the green alga spirogyra (Spirogyra weberi),
yeasts (Saccharomyces pombe), the scorpio (Tityus bahiensis), etc. The
genomes of the orders from 10 to 30 chromosomes are prevalent however.
Huge deviations are observed in amoeba (Amoeba proteus), the crawfish
(Astacus trowbridgei) and the snake fern (Ophioglossum petiolatum), where
the chromosome number is respectively 250, 188 and 520.
In Figure 2–20 is presented the cytological picture of a cell from
through hawksbeard (Crepis capilaris) with a diploid chromosome set
consisting of 6 chromosomes or 3 homologous pairs. Except diploid as was
already mentioned, the chromosome set can be triploid (3n), tetraploid (4n)
or polyploid.