CHROMOSOME NUMBERS AND THEIR ALTERATION 



211 



nature, and it has been found that the development of tetraploid indi- 

 \iduals can be induced in the laboratory by refrigerating eggs that would 

 normally have developed parthenogenetically into diploids. Another 

 notable case is a series of polyploid salamanders that appeared spon- 

 taneously in laboratory cultures (Fig. 157). A beginning has also been 

 made on the investigation of colchicine-induced chromosome doubling in 

 animal cells. 





izm 



Fig. 157. — Larvae of the common newt {Triturus viridcscens) with different numbers of 

 genomes. From left to right: pentaploid, tetraploid, triploid, diploid, and monoploid 

 specimens. Below them are mitotic metaphases observed in bits of tail fins stained and 

 mounted without sectioning. The normal diploid larva has 22 chromosomes. The size 

 of nuclei and cells increases roughly in proportion to the chromosome number, but the 

 body size does not; this indicates a lower cell number in the polyploids. (Courtesy of G. 

 Fankhausrr.) 



Other Types of Heteroploidy. — In the sporophytes of plants true 

 monoploidy occurs very rarely. Such plants have appeared in cultures in 

 the case of several angiosperm genera (Datura, Zea, Crepis, Nicotiana, 

 Oenothera, Triticum, and others), but nowhere have such monoploids 

 become estabUshed in nature, so far as we know. The main reason for 

 this appears to be their high degree of sexual sterility. Although they are 

 usually somewhat smaller and less vigorous than diploids, the}^ often 

 grow well, but at the time of meiosis the chromosomes of the single 

 genome, having no synaptic partners, behave so irregularly that practi- 

 cally no functional spores or gametes are produced. Rarely all the 

 chromosomes are included in a single spore, so that after many trials a 

 diploid plant is sometimes obtained by selfing a monoploid one. Such a 



