190 I The Process of Evolution 



distinction between polyploids in which all the genomes are alike, 

 or autopolyploids, and polyploids in which the genomes are differ- 

 ent, allopolyploids. In practice, this distinction may be difficult to 

 draw. Clearly, organisms are autopolyploid when they are the result 

 of somatic doubling of chromosome number (unless the parental 

 organism is a diploid hybrid). Usually plants resulting from the 

 fusion of haploid or polyploid gametes of the same species are re- 

 garded as autopolyploids. The genomes, if not the genes, are pre- 

 sumably much the same. The difficulty arises when wider crosses 

 are involved or when hybrids experience doubling, leading to com- 

 plex combinations of autopolyploidy and .allopolyploidy. 



Among the higher plants, sporophytic haploids with the gametic 

 number of chromosomes ( where n = x) in their somatic tissues are 

 unknown in natural populations. They may be produced in culture 

 and are of great interest cytogenetically but they are not known to 

 be of direct evolutionarv significance. 



Strict autopolyploids also are rare in nature. The presence of more 

 than two similar genomes in an autotriploid (3x) or autotetraploid 

 (4x) organism leads to difficulties at meiosis: Only two homologous 

 chromosomes can synapse at any one point. Instead of the usual 

 bivalents, multivalents and/or univalents may be found, depending 

 upon chance and the length of the chromosomes as related to chi- 

 asma number. Although numerous autopolyploid organisms may 

 show enhanced "vigor" or other physiological properties considered 

 advantageous by the plant breeder, they ordinarily are unable to 

 reproduce sexually and are selected against in nature unless they 

 acquire an efficient mode of asexual reproduction (see below). The 

 reduction in fertility may not be complete (it may be very slight); 

 furthermore, most of the higher plants have one or more modes of 

 vegetative propagation. Many of the most important horticultural 

 and pomicultural plants are triploids (such as most bananas, many 

 apples, some cherries, Japanese iris, tiger lilies, tulips). In addition, 

 both autotriploids and autotetraploids occasionally occur spontane- 

 ously in animals (e.g., in certain salamanders). Autopolyploids 

 rarely become established as populations in nature, however. 



Allopolyploidy, on the other hand, is known in virtually every 

 phylum of the plant kingdom. Well-known and often-discussed 

 examples are the allotetraploid of radish and cabbage, Raphano- 

 hrassica, and the allotetraploid of two horticultural species of prim- 

 rose, Frimula kewensis. The first involves a cross between different 

 genera, the second different species; therefore the two genomes in 

 the hybrid would most certainly be different. 



In diploid hybrids between species and genera, chromosome be- 



