POLYPLOIDY 237 



more productive, and again, those with an even number of chromo- 

 somes are better fruit bearers than those with an odd number. 

 For example, the tetraploids, hexaploids and octoploids are more 

 productive than triploids, pentaploids and heptaploids. 



Fertihty in the genus Rubus, for example, is closely associated 

 with a balanced chromosome constitution. The basic chromo- 

 some number here is seven. The raspberry varieties, Superlative 

 and Lloyd George, have 14 chromosomes, while Mahdi and Veitch- 

 berry have 21 and 28 respectively and Loganberry and Laxtonberry 

 42 and 49 respectively. It is a well-known fact of horticulture 

 that the triploid Mahdi and the heptaploid Laxtonberry are poorer 

 croppers than the even chromosome series, the diploid raspberry 

 the tetraploid Veitchberry and the hexaploid Loganberry. Excep- 

 tions to this rule are due to apogamous development of seed. 



The chances of obtaining fertile progeny from cross-breeding is 

 largely limited by the numerical relationships of the chromosome 

 complements. If a diploid be crossed with a tetraploid the 

 offspring are triploid and a tetraploid crossed with a hexaploid 

 yields pentaploids. It has been shown by Crane and others, 

 however, that on occasion exceptional seedlings do arise from such 

 matings. Thus when Rubus rusticanus inermis (diploid species) 

 with 14 chromosomes was crossed with Rubus thyrsiger (tetraploid 

 species) with 28 chromosomes, three seedlings were obtained, two 

 of which were triploids arising from the addition of seven rusti- 

 canus and 14 thyrsiger chromosomes. The other seedling had 

 28 chromosomes and arose by the functioning of an unreduced 

 Qgg of its mother, wiiereby the full chromosome complement of 

 rusticanus (14) combined with the haploid complement of thyrsiger 

 (14) yielded a tetraploid plant with 28 chromosomes. This tetra- 

 ploid seedling proved to be highly fertile and very productive, 

 whereas only occasional druplets were obtained from the triploid 

 offspring. It would seem that the Veitchberry and Laxtonberry 

 have originated from the functioning of a gamete with the unre- 

 duced number of chromosomes. For example, the Laxtonberry 

 was derived from the Loganberry x Superlative and has 49 chromo- 

 somes. Here there has been a combination of the full complement 

 of the Loganberry (42) with the haploid number of Superlative (7). 



