The basic chromosome number inPyrus is seventeen. Cultivated varieties are all orthoploid. Aneuploid seedlings are poor and abnormal.
The somatic chromosomes in “diploid”Pyrus have four representatives of a long type, in “triploid,” six.
Multiple association occurs amongst the chromosomes of “diploid”Pyrus giving, in extreme cases, seven groups; four quadrivalents and three sexivalents (Table I).
In “triploid” varieties ofP. Malus associations of four, five, six, seven, eight and nine chromosomes have been observed, although trivalents are usually formed (Table II). This means that antosyndesis takes place within each of the three supposed haploid complements.
Instead of giving a binomial frequency or the elimination of intermediate numbers, natural seedlings of “triploid” apples most frequently have numbers of chromosomes approximately to 2n + 7 (Table III).
Thus the pairing, morphology, and breeding results show, directly or indirectly, that the thirty-four chromosomes in the “diploid”Pyrus are of seven types, of which four are represented four times and three are represented six times. Such forms may be described as trebly hexasomic tetraploids (v. diagram, p. 145).
The number seventeen is therefore a secondary (unbalanced) basic number, and the derived series of polyploids (2n = 34, 51, 68) aresecondary polyploids.
The establishment of a secondary basic number must mean (on the analogy of all experimental observations) a definite evolutionary step. It is therefore plausible that thePyrus group owe their special morphological characters (e.g. the pome type of fruit) to this reorganisation of the nucleus. The work is being continued with this consideration in view.
Volume 100, 2021
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