Sex Chromosomes in Plants 



73 



parts only. If the homologous parts are not too short, chiasmata 

 are formed, and genes are exchanged between the two chromo- 

 somes. Since nonhomologous parts do not pair, there is no 

 exchange of segments between them and no chiasmata are formed, 

 but the presence of large 

 nonhomologous regions does 

 not prevent pairing in ho- 

 mologous parts. 



Sex Chromosomes in Plants 



Most seed plants are mo- 

 noecious — that is, both sexes 

 are present on each plant. 

 There is no sex chromosome 

 mechanism, and sex is not a 

 problem of heredity but one 

 of differentiation during de- 

 velopment. In some species 

 of plants, however, each in- 

 dividual is either male or 

 female. In some of these 

 plants a sex mechanism has 

 been discovered like that in 

 animals. In Lychnis dioica, 

 G. H. Shull showed from ge- 

 netic grounds that the male 

 was heterogametic. Defi- 

 nite X and Y chromosomes 

 have been found in Elodea 

 canadensis, Melandrium al- 

 bum (which is partially syn- 

 onymous with Lychnis dioica 



since it was included in ShulFs L. dioica), hops, poplar, and 

 other plants (Fig. 24) ; inheritance in these plants is of the 

 Drosophila type. One seed plant, Fragaria elatior, is of the 

 Abraxas type. In species of the dock, Rumex, the male is hetero- 

 gametic for sex, but has two small Y chromosomes and one large 

 X. The two Y chromosomes separate from the X at meiosis, and 

 a male gamete with two Y's produces a male on fertilization 

 whereas one with the one X produces a female. 



Fig. 24. Metaphase of first mei- 

 otic division in a diploid male plant 

 of Lychnis (Melandrium). Eleven 

 pairs of autosomes are present and 

 one pair which consists of the X and 

 Y chromosomes. Photomicrograph 

 X1400. (Courtesy of Dr. H. E. 

 Warmke in the American Journal of 

 Botany.) 



