ORIGIN OF TRIPLOIDS 67 



These are parthenogenetic in the tetraploid form when sexual in the 

 diploid (v. Ch. XI). Polyploidy can be inferred from their chromo- 

 some numbers in the hermaphrodite Mollusca and Annelida. 



This suggests that the rarity of tetraploid animal forms is due to 

 the upset of the chromosome mechanism of sex differentiation, as 

 has been found in Empetnim (Hagerup, 1927) where a diploid 

 species is dioecious while its tetraploid relative is hermaphrodite. 

 It may be thought, on the other hand, that the closed growth 

 system of animals is responsible for the absence of polyploids, but, 

 while this is a factor which will operate against their occurrence, it 

 does not account for the complete absence of sexually reproducing 

 polyploids (cf. Muller, 1925 h). 



The nuclei of the endosperm in the flowering plants are triploid 

 through the fusion of three nuclei, but such is not the origin of most 

 triploid plants in experiment and in nature. One method of origin 

 is by an abnormality in somatic mitosis in the haploid generation. 

 Newton (1927) found nuclei at the third division of the embryo-sac 

 with the diploid number of chromosomes, instead of the haploid, 

 in many species of the Leiostemones section of Tulipa, of which this 

 behaviour appears to be a characteristic. The doubling sometimes 

 occurs regularly and presumably through the failure of anaphase 

 separation at the division of one of the first two daughter nuclei in 

 the embryo-sac (the second meiotic division). When it happens 

 at the antipodal end it will result in the formation of endosperm 

 with the tetraploid instead of the usual triploid number. When 

 it happens, as it does exceptionally, at the egg-cell end, the embryo 

 resulting from fertilisation will be triploid. This aberration there- 

 fore accounts for the occurrence of triploid forms and species in 

 Tulipa and perhaps elsewhere in the Liliaceae where non-hybrid 

 triploids are common (v. Ch. V). 



Triploids are probably derived more often through failure of 

 reduction at meiosis {q.v.) on one side, and the consequent fusion of 

 reduced {n) and unreduced {2n) gametes. In this way triploids 

 have been produced in Ascaris (Boveri), Drosophila (Bridges, 192 1 a), 

 and in PygcBva hybrid back-crosses (Federley, 1912, 1931). This 

 method of origin is particularly frequent in the dicotyledons (Ch. 

 VII). 



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