TRENDS IN SYSTEMATIC BOTANY
67
taxonomy. If the chromosomes are large enough to be observed readily,
they may show constant differences in length at mitotic metaphase and in
shape as marked by the presence of one or more constrictions along
their length. If a chromosome has but one constriction (the so-called
spindle attachment region), it will be marked by two arms of constant
length. Secondary constrictions may occur, and if one is subterminal it
may set off a very small segment of chromosome called a satellite, which,
when present, is also a constant feature. In some groups of plants these
features are easily recognizable and of considerable use in differentiating
taxa (P. C. Bailey, 1951; Britton, 1951; Cave, 1948; Cleland, 1937, 1944;
Granick, 1944; Levan, 1944; McKelvey and Sax, 1933; Simonet, 1932;
Uhl, 1948).
Chromosome behavior may be of profound importance in determining
taxonomic relationships, for the kind of pairing that takes place in a
plant at meiosis may indicate its hybrid origin, or reveal structural differences in the chromosomes of its parents, or explain its observed sterility. The study of chromosome homology in a hybrid (homologous
chromosomes pair at meiosis) gives some measure of the degree of relationship between its parents. Suene (Caryophyllaceae) is a genus
of some 54 North American species, with many more in the Old World.
Eight of the former group, 4 tetraploids, 1 hexaploid, and 3 octoploids,
were successfully hybridized in 12 different combinations by Kruckeberg
(1955). Despite the high incidence of crossability within this random
group of well-marked species, the hybrids showed a uniformly low
fertility. This was closely correlated with irregularities in chromosomal
pairing at meiosis which pointed to a considerable lack of chromosome
homology among the parents.
During the evolutionary process increases in chromosome number can
occur in various ways, one example being through fragmentation of an
existing chromosome. It is generally accepted that an increase in
chromatin material is less deleterious to the plant than a decrease, and
that accordingly it is much easier for a taxon to arise from another with
a lower chromosome number than vice versa. It is obvious, however, that
low chromosome numbers are found throughout the plant kingdom, with
the lowest, three, found in Crepis, one of the most specialized genera in
the most advanced family, Compositae. A number of examples are now
on record that illustrate how a decrease in chromosome number may
arise. It is often through an unequal interchange between two chromosomes, with a resultant loss of the smaller. This phenomenon may have
happened rather commonly among plants. The best known examples are
provided in Crepis (Tobgy, 1943; Babcock, 1947c), with additional ex-
67
taxonomy. If the chromosomes are large enough to be observed readily,
they may show constant differences in length at mitotic metaphase and in
shape as marked by the presence of one or more constrictions along
their length. If a chromosome has but one constriction (the so-called
spindle attachment region), it will be marked by two arms of constant
length. Secondary constrictions may occur, and if one is subterminal it
may set off a very small segment of chromosome called a satellite, which,
when present, is also a constant feature. In some groups of plants these
features are easily recognizable and of considerable use in differentiating
taxa (P. C. Bailey, 1951; Britton, 1951; Cave, 1948; Cleland, 1937, 1944;
Granick, 1944; Levan, 1944; McKelvey and Sax, 1933; Simonet, 1932;
Uhl, 1948).
Chromosome behavior may be of profound importance in determining
taxonomic relationships, for the kind of pairing that takes place in a
plant at meiosis may indicate its hybrid origin, or reveal structural differences in the chromosomes of its parents, or explain its observed sterility. The study of chromosome homology in a hybrid (homologous
chromosomes pair at meiosis) gives some measure of the degree of relationship between its parents. Suene (Caryophyllaceae) is a genus
of some 54 North American species, with many more in the Old World.
Eight of the former group, 4 tetraploids, 1 hexaploid, and 3 octoploids,
were successfully hybridized in 12 different combinations by Kruckeberg
(1955). Despite the high incidence of crossability within this random
group of well-marked species, the hybrids showed a uniformly low
fertility. This was closely correlated with irregularities in chromosomal
pairing at meiosis which pointed to a considerable lack of chromosome
homology among the parents.
During the evolutionary process increases in chromosome number can
occur in various ways, one example being through fragmentation of an
existing chromosome. It is generally accepted that an increase in
chromatin material is less deleterious to the plant than a decrease, and
that accordingly it is much easier for a taxon to arise from another with
a lower chromosome number than vice versa. It is obvious, however, that
low chromosome numbers are found throughout the plant kingdom, with
the lowest, three, found in Crepis, one of the most specialized genera in
the most advanced family, Compositae. A number of examples are now
on record that illustrate how a decrease in chromosome number may
arise. It is often through an unequal interchange between two chromosomes, with a resultant loss of the smaller. This phenomenon may have
happened rather commonly among plants. The best known examples are
provided in Crepis (Tobgy, 1943; Babcock, 1947c), with additional ex-