TRENDS IN SYSTEMATIC BOTANY
73
An interesting case is found in Tragopogón
(Compositae), of which
three species have been naturalized in America in the Pacific Northwest
(M. Ownbey, 1950). Natural hybrids between them can be found where
they grow together, and these have but low fertility. However, four
small amphiploid colonies have been found which are fertile and apparently true breeding. Genetically, at least, these must be considered
new species. Since they have undoubtedly arisen in recent times, if undisturbed they may be expected to modify their biology somewhat as
time goes on, perhaps in the direction of increasing both their fertility
and their range.
Because of the economic importance of the cotton plant,
Gossypium,
it and its relatives are among the most thoroughly studied groups from
the cytological viewpoint. The taxonomy of these genera, and particularly of the species of Gossypium, has been strongly influenced by the
cytological and genetical data available. One indication of the reliability
of such data is that Skovsted (1934, 1937) proposed on cytological and
morphological grounds a hypothesis that seemed very bold at the time
when viewed phytogeographically, namely, that the New World cottons originated as amphiploids from a hybrid between Asiatic cotton and
a wild species from America, and this hypothesis was subsequently
verified experimentally (Beasley, 1940).
From the cytological viewpoint the genome, or basic haploid set of
chromosomes of a diploid species, is the next higher fundamental evolutionary unit above the chromosome. A polyploid species contains more
than one genome in its sex cells. Upon hybridization a new combination
of genomes takes place. If the chromosomes that are thus brought together are able to find partners and chromosome pairing is regular, one
possible source of disharmony in the development of the hybrid is removed. Hence the study of genome behavior is evolutionarily significant.
In Gossypium the cytological affinity of each genome with every other in
the genus has been studied (Brown and Menzel, 1952). The relationships established by this method are in excellent agreement with those
proposed from other kinds of evidence. It was demonstrated that exchange of chromatin was cytologically possible among all genome groups
in the genus, although fertility barriers were in the way. Theoretically,
however, further evolutionary progress in Gossypium is possible along
this line.
It may be enlightening to the taxonomist to consider several examples
of polyploidy as it bears on speciation. The Sanícula crassícaulis complex
has unusual aspects (Bell, 1954). Three diploid species of relatively restricted distribution are believed to be the primary taxa involved in this
73
An interesting case is found in Tragopogón
(Compositae), of which
three species have been naturalized in America in the Pacific Northwest
(M. Ownbey, 1950). Natural hybrids between them can be found where
they grow together, and these have but low fertility. However, four
small amphiploid colonies have been found which are fertile and apparently true breeding. Genetically, at least, these must be considered
new species. Since they have undoubtedly arisen in recent times, if undisturbed they may be expected to modify their biology somewhat as
time goes on, perhaps in the direction of increasing both their fertility
and their range.
Because of the economic importance of the cotton plant,
Gossypium,
it and its relatives are among the most thoroughly studied groups from
the cytological viewpoint. The taxonomy of these genera, and particularly of the species of Gossypium, has been strongly influenced by the
cytological and genetical data available. One indication of the reliability
of such data is that Skovsted (1934, 1937) proposed on cytological and
morphological grounds a hypothesis that seemed very bold at the time
when viewed phytogeographically, namely, that the New World cottons originated as amphiploids from a hybrid between Asiatic cotton and
a wild species from America, and this hypothesis was subsequently
verified experimentally (Beasley, 1940).
From the cytological viewpoint the genome, or basic haploid set of
chromosomes of a diploid species, is the next higher fundamental evolutionary unit above the chromosome. A polyploid species contains more
than one genome in its sex cells. Upon hybridization a new combination
of genomes takes place. If the chromosomes that are thus brought together are able to find partners and chromosome pairing is regular, one
possible source of disharmony in the development of the hybrid is removed. Hence the study of genome behavior is evolutionarily significant.
In Gossypium the cytological affinity of each genome with every other in
the genus has been studied (Brown and Menzel, 1952). The relationships established by this method are in excellent agreement with those
proposed from other kinds of evidence. It was demonstrated that exchange of chromatin was cytologically possible among all genome groups
in the genus, although fertility barriers were in the way. Theoretically,
however, further evolutionary progress in Gossypium is possible along
this line.
It may be enlightening to the taxonomist to consider several examples
of polyploidy as it bears on speciation. The Sanícula crassícaulis complex
has unusual aspects (Bell, 1954). Three diploid species of relatively restricted distribution are believed to be the primary taxa involved in this