68
DAVID D. KECK
ampies from other Cichorieae (Stebbins et al. 1953),
Polygonatum
(Therman, 1953), Clarkia (Lewis, 1953b), and Amsinckia (Ray, 1934).
It has been shown in Campanula
(Darlington and La Cour, 1950) that
potentially either an increase or decrease in chromosome number is
possible in species in which interchange heterozygotes occur. In this
genus fertile individuals with 9 pairs of chromosomes were derived from
parental types with 8 when a plant became homozygous for 2 complementary chromosomes derived from misdivision of the centromere, and a
monosomic plant arose that was deficient for a new interchanged chromosome. If such deficiencies succeed in nature, a new basic chromosome
number may be established.
Polyploidy.
A polyploid is an organism whose chromosome number
is a multiple of any basic number found (or thought to exist) in a related organism. An autoploid (autopolyploid)
may be defined as an organism having the chromosome sets within the limits of one species
multiplied, an alloploid (allopolyploid)
is a polyploid with chromosome
sets from difference sources, as from different species, and an amphiploid
is an organism having all the chromosomes from two distinct species
added. An amphiploid is a product of amphiploidy,
an inclusive term as
used by Clausen et al. (1945) to include amphidiploidy
and allopolyploidy. Amphidiploidy was first used by Navaschin (1927) to describe
the process giving rise to the hypothetical doubled hybrid of Crepis capillaris (n = 3) X C. setosa (n = 4). Since the haploid chromosome
number of such a doubled hybrid would be 7, which is not a multiple of
any basic number, it could scarcely be called a "polyploid." An allopolyploid, on the other hand, may be an amphidiploid in some cases if its
chromosome complement falls in a polyploid series with its parental
species through the addition or multiplication of their base numbers,
but it could not properly be termed an amphidiploid if, due to partial
or complete intergenomal pairing, it does not behave cytologically at
all like a diploid.
The significance of polyploidy in the evolution of plants is widely
recognized, but the subject is not a simple one and there has been considerable development of thought on the relative roles of the different
kinds of polyploidy in speciation. "About half of the species of Angiosperms have chromosome numbers that clearly indicate polyploid origin,
and in some families, like the Gramineae, three-fourths of the species are
polyploids" (Stebbins, 1947). As an example of its importance one may
point to the Malvaceae, with interspecific polyploidy very frequent and
intraspecific polyploidy also a factor (Skovsted, 1941). This family has
developed at least five chromosome series: (a) a five series (5, 10, 15, 20,
DAVID D. KECK
ampies from other Cichorieae (Stebbins et al. 1953),
Polygonatum
(Therman, 1953), Clarkia (Lewis, 1953b), and Amsinckia (Ray, 1934).
It has been shown in Campanula
(Darlington and La Cour, 1950) that
potentially either an increase or decrease in chromosome number is
possible in species in which interchange heterozygotes occur. In this
genus fertile individuals with 9 pairs of chromosomes were derived from
parental types with 8 when a plant became homozygous for 2 complementary chromosomes derived from misdivision of the centromere, and a
monosomic plant arose that was deficient for a new interchanged chromosome. If such deficiencies succeed in nature, a new basic chromosome
number may be established.
Polyploidy.
A polyploid is an organism whose chromosome number
is a multiple of any basic number found (or thought to exist) in a related organism. An autoploid (autopolyploid)
may be defined as an organism having the chromosome sets within the limits of one species
multiplied, an alloploid (allopolyploid)
is a polyploid with chromosome
sets from difference sources, as from different species, and an amphiploid
is an organism having all the chromosomes from two distinct species
added. An amphiploid is a product of amphiploidy,
an inclusive term as
used by Clausen et al. (1945) to include amphidiploidy
and allopolyploidy. Amphidiploidy was first used by Navaschin (1927) to describe
the process giving rise to the hypothetical doubled hybrid of Crepis capillaris (n = 3) X C. setosa (n = 4). Since the haploid chromosome
number of such a doubled hybrid would be 7, which is not a multiple of
any basic number, it could scarcely be called a "polyploid." An allopolyploid, on the other hand, may be an amphidiploid in some cases if its
chromosome complement falls in a polyploid series with its parental
species through the addition or multiplication of their base numbers,
but it could not properly be termed an amphidiploid if, due to partial
or complete intergenomal pairing, it does not behave cytologically at
all like a diploid.
The significance of polyploidy in the evolution of plants is widely
recognized, but the subject is not a simple one and there has been considerable development of thought on the relative roles of the different
kinds of polyploidy in speciation. "About half of the species of Angiosperms have chromosome numbers that clearly indicate polyploid origin,
and in some families, like the Gramineae, three-fourths of the species are
polyploids" (Stebbins, 1947). As an example of its importance one may
point to the Malvaceae, with interspecific polyploidy very frequent and
intraspecific polyploidy also a factor (Skovsted, 1941). This family has
developed at least five chromosome series: (a) a five series (5, 10, 15, 20,