THE ORIGIN OF SPECIES 



prominent group, it is likely to consist of one or a few polyploid species 

 without near relatives. Thus the entire order Psilotales, once a dominant 

 group, is now represented by only two genera, which are regarded by 

 many botanists as monotypic. These have over a hundred pairs of chromo- 

 somes, and so it is almost certain that they are the last remnants of a once 

 great polyploid complex. 



POLYPLOIDY IN THE ANIMAL KINGDOM 



Although polyploidy has been a major phenomenon in plant evolution, 

 its role in animal evolution has not been adequately assessed, and it is 

 generally considered to be of minor importance. The reason for this dif- 

 ference between the kingdoms is not known with certainty, but Muller 

 has suggested that it may be based upon the fact that the sexes are usually 

 separate in animals, while plants are usually hermaphroditic (mono- 

 ecious). Random segregation of the several pairs of sex chromosomes in 

 a polyploid organism would result in sterile combinations. This explana- 

 tion has been widely accepted. Vandel has reviewed all of the known 

 cases of polyploidy in animals, and the data which he has assembled lend 

 support to Muller's theory. Thus he finds, among plants, only eleven cases 

 in which polyploid plants are also dioecious (sexes separate). These in- 

 clude Fragaria elatior, a hexaploid species of strawberry. It has been 

 proven that there is only one pair of sex chromosomes present in this spe- 

 cies. Whether the other two pairs have lost their original sex-differenti- 

 ating function, or whether polyploidy first developed in a monoecious 

 ancestor, with the separation of the sexes occurring later, cannot be ascer- 

 tained. Vandel favors the former hypothesis. In any event, it appears that 

 polyploidy, so common among plants in general, is rare among those 

 plants which have acquired the dioecious habit. 



The majority of the animals which have been reported to be polyploids 

 are parthenogenetic. Curiously, all of these parthogenetic polyploids are 

 arthropods. The common waterflea, Daphnia pidex, occurs in a diploid, 

 sexually reproducing form, and in a hexaploid, parthenogenetically repro- 

 ducing form. Artemia salina, the brine shrimp, occurs in tetraploid and 

 octoploid parthenogenetic forms, but there is also a tetraploid race which 

 reproduces sexually. The European sow bug, Trichoniscus cUsahetJme, is 

 a triploid parthenogenetic species, while it is uncertain whether the par- 

 thenogenetic ostracod, Cypris juscata, is triploid or tetraploid. The "walk- 

 ing stick" insects, Carausius morosus and C. furciUatiis are, respectively, 

 triploid and tetraploid, as well as parthenogenetic. The psychid moths 

 Solcnohia triquetrclla and S. lichenella are also parthenogenetic tetra- 

 ploids, but a bisexual, diploid race of the former is known. Finally, the 

 parthenogenetic beetle, Trachijphlaeus, also appears to be tetraploid. 



A very few tetraploid animals reproduce bisexually. Artemia has already 

 been mentioned. Parascaris eqiiorum ( = Ascaris megalocephala of older 

 literature), an important nematode parasite of horses, is known in dip- 

 loid, tetraploid, and hexaploid forms, all of which reproduce bisexually. 

 The starfishes Asterias forhesii and A. glackilis and the sea urchin Echinus 



306 



