238 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



TSHflWTTSCMA 



Figure 2. — Relative success of crossbreeding of the five 

 eastern Pacific species of Onchorhynchus. (Lines in- 

 dicate relative sucess ; see table 3. Arrows indicate 

 direction of male-female cross. ) 



This figure sliows S. trutta occupying a position 

 between the Salvelinae and the other species of 

 Salmo, approaching closest to S. salar. The Sal- 

 velinae appear to be a closely knit group, but not 

 Salmo. It is surprising that t)'Utta will hybridize, 

 despite the difference in chromosome number with 

 both salar and gairdneri^ yet the latter two so far 

 appear incompatible. No one has been successful 

 in crossing a male S. gairdneri with the female of 



another species, which suggests incompatibility of 

 the male sex chromosome. 



The only experiments in crossing Oncorhynchus 

 with other Salmonidaj were those of Roosevelt 

 (1880) and Green (1881). In both cases male O. 

 tshawytscha from eggs taken in the Sacramento 

 River system were crossed with female *S'. fontina- 

 7m, and in both cases hybrids were raised to ma- 

 turity, but the hybrids were all females, and the 

 eggs would not hatch when fertilized with milt 

 from male S. fontinalis. 



Within the genus Oncorhynchus all five species 

 were crossed in both directions by Foerster ( 1935) ; 

 his results are summarized in table 3 and figure 2. 



From figure 2, in which the length of each line 

 coincides with the subjective rating of table 3, it 

 is clear that klsvtch is rather apart from the re- 

 mainder of the species. This seems to coincide 

 with the conclusions of Milne (1948) from a study 

 of certain morphological characters which will be 

 discussed later. Natural hybrids of keta and 

 gorhiischa are not uncommon, and Hunter (1949) 

 describes the examination of about 50 such hybrids 

 at Port John, British Columbia; other natural 

 crosses are more rare. The contribution of 

 hybridization toward understanding relationships 

 will have to be evaluated together with other 

 characteristics. 



Table 2. — Some interspecific crosses in Salmonidae 



[Excepting Oncorhynclius] 



' Subjective ratings of relative success: 1, excellent; 2, good; 3, moderate; 4, poor; 6, very poor; 6, failure. 



