CYTOGENETICS OF RUBUS. II. CYTOLOGICAL STUDIES OF THE VARIETIES ‘YOUNG,’ ‘BOYSEN,’ AND RELATED FORMS

T hompson , M axine M. (U. California, Davis.) Cytogenetics of Rubus. II. Cytological studies of the varieties ‘Young,’ ‘Boysen’ and related forms. Amer. Jour. Bot. 48(8): 667–673. Illus. 1961.—Chromosome numbers are given for the trailing blackberry varieties, ‘Young’ (2 n = 49), ‘Boysen’ (2 n = 49), ‘Nectar’ (2 n = 49) and for related forms which include the parents of ‘Young,’ ‘Phenomenal’ (2 n = 42) and ‘Mayes’ (2 n = 56), and 3 cytologically resynthesized ‘Young’ plants (2 n = 49) as a basis for interpreting the postulated origin of ‘Young.’ Cytological evidence substantiated the conclusion that ‘Young’ is a hybrid between ‘Phenomenal’ and ‘Mayes.’ Contributions to the understanding of genomic relationships in Rubus are offered from detailed analyses of meiosis in ‘Phenomenal,’ ‘Mayes,’ ‘Young,’ and ‘Boysen.’ ‘Phenomenal’ and ‘Mayes’ both had a very regular meiosis. ‘Young,’ as well as ‘Boysen,’ showed a greater degree of chromosome association than either parent of ‘Young.’ Meiotic behavior in ‘Boysen’ presented a close parallel to that of ‘Young’ which, correlated with morphological similarities and the same 2 n chromosome number, suggests a similar origin. The mode of reproduction in ‘Young’ and ‘Boysen’ was found to be sexual on the basis of morphological variation in the open‐pollinated (selfed) progeny, the varying aneuploid somatic chromosome numbers in these progeny (2 n = 32–54) and aneuploid chromosome numbers in hybrids having either variety as one parent. The productiveness of ‘Young’ and ‘Boysen’ in commercial plantings and their successful utilization in breeding programs indicate a high fertility regardless of their having an odd multiple of the basic number. It is concluded that the production of balanced euploid gametes is not necessarily a criterion of fertility, at least not at this high level of ploidy.