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DAVID D. KECK
juvenated and "aggressive" through polyploidy, how autoploids and
amphiploids may recombine their characteristics, how the barriers between genetically separated diploid species break down when polyploids arise from them, and how species may have a polyphyletic origin
(Clausen et al 1940a).
Both autoploids and amphiploids occur in the heterophyllous asters
(Avers, 1953a, b). Crosses between the various taxa and chromosome
levels showed that crossability was higher at the tetraploid level than at
the diploid level, that the autoploids were always intraspecific polyploids, and that when the autoploids were used as parents in crossings
with other polyploids, very many recombination types resulted. In spite
of potentially extensive gene exchange in this group, the species retain
their individuality through the operation of other restrictive factors.
The European blackberries, genus Rubus, § Eubatus, form a single
enormous agamic complex, characterized by morphological diversity,
ecological differentiation, and a great capacity for multiplication. In
Sudre's monograph of them he treated in the Moriferi veri 109 principal
species, around which were clustered 691 species of more doubtful value
and 1927 varieties (Sudre, 1908-1913). In his index, 3350 names that
had been applied to species, varieties, and hybrids in this group were
enumerated! New biotypes are constantly arising through hybridization. Most of them should be regarded as amphiploids, but cytologically many show a rather high frequency of multivalents, i.e., they
simulate autoploids. They do not form a continuous series from one
extreme to the other, but on the contrary some 75 to 100 types can be
taxonomically distinguished rather easily. There are some southern,
sexual, diploid species, but the major number of the blackberries are
apomicts. The apomicts, however, have not lost their capacity for sexual
seed production (facultative apomixis) and help maintain an equilibrium system between apomixis and sexuality in the genus (Gustafsson,
1942, 1943).
The rubber-producing Composite, guayule, Parthenium
argentatum,
and the morphologically very different sister species, Parthenium
incanum (called mariola) occur together over much the same range in
western Texas and northern Mexico. There is ample opportunity for
hybridization between them, but the rate of this is very low as determined in cultivated plants. Introgression of genes is evident in the wild
species. Both have a polyploid series of high chromosome numbers based
on 9: in guayule, 2n = 36, 54, 58(2), 72, 74, and 108-112; and in mariola,
2n = 36, 54, 72, 81, and 90 (Rollins, 1944). In P. argentatum the 36chromosome plants are sexual, and those at higher chromosome levels are
DAVID D. KECK
juvenated and "aggressive" through polyploidy, how autoploids and
amphiploids may recombine their characteristics, how the barriers between genetically separated diploid species break down when polyploids arise from them, and how species may have a polyphyletic origin
(Clausen et al 1940a).
Both autoploids and amphiploids occur in the heterophyllous asters
(Avers, 1953a, b). Crosses between the various taxa and chromosome
levels showed that crossability was higher at the tetraploid level than at
the diploid level, that the autoploids were always intraspecific polyploids, and that when the autoploids were used as parents in crossings
with other polyploids, very many recombination types resulted. In spite
of potentially extensive gene exchange in this group, the species retain
their individuality through the operation of other restrictive factors.
The European blackberries, genus Rubus, § Eubatus, form a single
enormous agamic complex, characterized by morphological diversity,
ecological differentiation, and a great capacity for multiplication. In
Sudre's monograph of them he treated in the Moriferi veri 109 principal
species, around which were clustered 691 species of more doubtful value
and 1927 varieties (Sudre, 1908-1913). In his index, 3350 names that
had been applied to species, varieties, and hybrids in this group were
enumerated! New biotypes are constantly arising through hybridization. Most of them should be regarded as amphiploids, but cytologically many show a rather high frequency of multivalents, i.e., they
simulate autoploids. They do not form a continuous series from one
extreme to the other, but on the contrary some 75 to 100 types can be
taxonomically distinguished rather easily. There are some southern,
sexual, diploid species, but the major number of the blackberries are
apomicts. The apomicts, however, have not lost their capacity for sexual
seed production (facultative apomixis) and help maintain an equilibrium system between apomixis and sexuality in the genus (Gustafsson,
1942, 1943).
The rubber-producing Composite, guayule, Parthenium
argentatum,
and the morphologically very different sister species, Parthenium
incanum (called mariola) occur together over much the same range in
western Texas and northern Mexico. There is ample opportunity for
hybridization between them, but the rate of this is very low as determined in cultivated plants. Introgression of genes is evident in the wild
species. Both have a polyploid series of high chromosome numbers based
on 9: in guayule, 2n = 36, 54, 58(2), 72, 74, and 108-112; and in mariola,
2n = 36, 54, 72, 81, and 90 (Rollins, 1944). In P. argentatum the 36chromosome plants are sexual, and those at higher chromosome levels are