448 



Haploids and Autopolyploids 



the tetraploid form of Setcreasia brevifolia, another member of 

 the Tradeseantiae, meiotic behavior was very similar to that 

 of Tradescantia virginiana. The average number of quadriva- 

 lents per nucleus and the size and form of the metaphase con- 

 figurations were very similar in the two species. On the other 

 hand, in autotetraploid tomatoes there were a number of quad- 

 rivalent configurations in prophase 

 of the first meiotic division, but 

 they broke up into bivalents by 

 late diakinesis or metaphase so 

 that at metaphase twenty-four 

 bivalents lined up on the equator. 

 Thus the absence of quadrivalents 

 is not a sure sign that a plant is 

 not an autotetraploid. 



The segregation of genes in 

 autotetraploids is interesting. If 

 we assume that any one of the 

 arms can pair with the homolo- 

 gous arm of any of the other 

 three chromosomes, random pair- 

 ing among the four chromosomes 

 is attained. Random pairing 

 should result in random disjunc- 

 tion such that if one particular 

 chromosome goes to a given daugh- 

 ter nucleus, it will be accompan- 

 ied by any one of the other three with equal frequency. There- 

 fore, if the genes on the four chromosomes are AAaa, there 

 are equal chances that the gametes will contain the two ^'s, the 

 two a's, the first A and first a, the second A and second a, the 

 first A and second a, and the second A and first a. The gametic 

 ratio from such a tetraploid v/ill be lAA : 4iAa : laa. Similarly, 

 a plant whose genotype is Aaaa would have the gametes lAa : 

 laa, whereas one that is AAAa would produce gametes in the 

 ratio of lAA : lAa. If an autotetraploid has one dominant gene, 

 we say that it is simplex for that gene, whereas if it has two or 

 three, we refer to it as duplex or triplex, respectively. A plant 

 with no dominant genes is nulliplex; one with four dominants is 

 quadruplex. 



Fig. 128. Chromosomes at 

 first metaphase in a tetraploid 

 Tradescantia. Two quadriva- 

 lents and eight bivalents are 

 present. Left, two bivalents; 

 next left a figure-of-8 quadri- 

 valent with a bivalent below; 

 the other quadrivalent is at the 

 extreme right. 



