442 Haploids and Autopolyploids 



studied by King in 1933. He found that about 90 per cent of 

 the figures were trivalents with only about 10 per cent bivalents 

 and univalents. 



Because of the presence of an extra set of chromosomes, it is 

 easy to understand why triploids are so highly sterile. As we 

 pointed out for trisomies, whether the configuration is a trivalent 

 or a bivalent and a univalent, two of the daughter cells at the 

 first meiotic division receive two of the homologues whereas the 

 other cell receives one. Obviously, if each trivalent (or univa- 

 lent) segregates independently of every other, all types of 

 gametes from n to 2n should result and should be distributed 

 in the form of a frequency curve, with the n and 2n types least 

 frequent and the intermediate unbalanced types most frequent. 

 As in the haploid, the frequency of the types should be expressed 

 by the expansion of the binominal (Yo + %)", where n is the 

 number of chromosomes in a genome. Triploids are known 

 in which lagging of chromosomes, dicentric chromatids and chro- 

 matid bridges, fragments, and other chromosome aberrations are 

 common. 



Triploidy in higher animals is not common, although triploids 

 are known among vertebrates in frogs and salamanders. Fank- 

 hauser studied 100 larvae of the newt, Triturus viridescens. 

 Chromosome counts from the epidermis of the tail fin showed 

 that 96 were diploids with 22 chromosomes, whereas 4 had 33 

 chromosomes and were undoubtedly triploids. A similar exami- 

 nation of 134 larvae of the salamander, Eurycea bislineata, 

 showed that 119 were diploid, 13 were triploid, and 2 were tetra- 

 ploid. 



Endosperm 



In discussing the life cycle of plants we pointed out that the 

 gametophyte generation is haploid and the sporophyte diploid. 

 There is one tissue, however, that is regularly and normally 

 triploid. This tissue is a new one in the evolution of plants, 

 for it is found only in the highest and most recently developed 

 group of plants, the angiosperms. This tissue is the endosperm 

 of the seed, which is not the megagametophyte as it is in gym- 

 nosperms but a structure that arises during double fertilization 

 by a fusion of one sperm nucleus with the two polar nuclei in 

 the center of the embryo sac. This tissue is an important one for 



