University of Califoniia Publications in Agricultural Scient'es [Vol. 2 



EVOLUTION OF THE CHROMOSOME COMPLEXES 



IN TRIFOLIUM 



The chromosome complexes in many genera are now studied Avith 

 the aim of tracing the relationship between the species and of finding 

 the way in which the evolution of the species has proceeded. Attempts 

 are also made to base the classification of species on chromosome 

 morphology. For Vicia Sveshnikova (1927) has worked out a key 

 ba.sed on chromosome morphology and finds that it corresponds 

 very nearly to the key worked out by Ascherson based on external 

 morphology. In TrifoJmm it is evident that there is no such paral- 

 lelism in the differentiation of the chromosome complexes and of the 

 external morphology of the species. Species which are far removed 

 taxonomically and very different in their morphology have very 

 similar chromosome complexes ; for instance, the European species, 

 T. glomeraium, and the Calif ornian species, T. ohfiisiflorum. On the 

 other hand, we find nearly related species with very different chromo- 

 some complexes. The wild red clover, T. prafense, is very similar to 

 T. medium, but the former has 14 and the latter about 130 chromo- 

 somes. Furthermore, though the number is the same, the shape and 

 the size of the chromosomes may be different. T. pratense and 

 T. incarnatum are placed in the same subsection of the section, 

 Eulagopus, but the chromosome complexes are very unlike. In T. 

 alexandrinum and T. incarnatum we have two species differing in 

 external morphology and in chromosome number (16 and 14) but 

 very similar as regards the shape and size of the chromosomes, 

 alexandrinum having an extra pair of medium sized chromosomes. 

 The situation in Trifolium suggests that it will not be easy on the 

 basis of chromosome morphology to trace the mutual relationship and 

 origin of the species in tliis genus. The basis for such a study must 

 be the possibility of establishing certain types of chromosomes, which 

 can be identified in related species. Nawaschin's (1925) work on the 

 genus Crepis is of this type. In ten species with 3, 4, and 5 pairs 

 of chromosomes he established five types of chromosomes, one of whicli 

 was a satellited chromosome. In the summary Nawaschin states, 

 "Es wurde von mir festgestellt dass dieselbe homologischen Typen 

 und Formen der Chromosomen in den Chromosomsiitzen aller unter- 

 suchten Arten hervortreten. " In Trifolium ten species at least have 



