TRENDS IN SYSTEMATIC BOTANY
85
by the percentage of pairing in their hybrids. The 7-chromosome species
all seem to have homologous or almost homologous complements of
chromosomes irrespective of the distinctness of the morphological and
genetical barriers in the group. On the other hand, while 5 of the 8chromosome species exhibit a high degree of chromosome homology,
as evidenced by almost perfect pairing in their hybrids, the other 4 show
little or no chromosome homology with these or among themselves.
There is furthermore almost no pairing of chromosomes in the rare hybrids between 7- and 8-chromosome species. This suggests that one
group has not evolved from the other through the simple addition or loss
of one chromosome.
The cytology also answers a question in regard to the position of the
16-chromosome L. panicufota, which morphologically stands so close to
L. hieracioides, its diploid counterpart, as to suggest an autoploid origin.
Homology is quite lacking among the chromosomes of this species in the
hybrid with 8-chromosome L. gaillardioides,
although the chromosomes
of L. gaillardioides
and L. hieracioides,
on the other hand, are highly
homologous. This indicates that L. paniculata is not of autoploid origin
from L. hieracioides, for in that case one would expect at least the 16
L. panicufota chromosomes to pair among themselves in such a hybrid.
The hybrid with L. gaillardioides
was the only one obtained using L.
panicufota as a parent. These cytological results suggest that L. panicufota has had an independent origin, possibly through amphiploidy, with
one at least of its ancestors now extinct. Despite its morphological
similarity to L. hieracioides, which caused it to be overlooked previously
as a taxon, Layia panicufota is shown by the experimental evidence to
have been amply distinct both cytologically and genetically probably
for a long time.
The genetic criteria of relationships among the species of Layia as
determined by crossability, the relative ease of gene exchange, and
fertility of F x hybrids and relative vigor of F 2 hybrids (a measure of
survival potential), show that the genus is composed of 5 species complexes or cenospecies. Three of these are monotypic, the other 2 are
composed of 6 species each. Using the terminology of the experimentalist,
all 15 species of Layia are ecospecies, but 3 of them are also monotypic
cenospecies.
It is revealing to compare the three kinds of discontinuity in the genus.
The 6 species of the group each of which has 7 pairs of chromosomes are
also shown by their crossability to belong to one cenospecies, so here the
cytological and genetical data agree. Morphologically there are 3 groups
among the 6 species, each being set off from the others by a strong
85
by the percentage of pairing in their hybrids. The 7-chromosome species
all seem to have homologous or almost homologous complements of
chromosomes irrespective of the distinctness of the morphological and
genetical barriers in the group. On the other hand, while 5 of the 8chromosome species exhibit a high degree of chromosome homology,
as evidenced by almost perfect pairing in their hybrids, the other 4 show
little or no chromosome homology with these or among themselves.
There is furthermore almost no pairing of chromosomes in the rare hybrids between 7- and 8-chromosome species. This suggests that one
group has not evolved from the other through the simple addition or loss
of one chromosome.
The cytology also answers a question in regard to the position of the
16-chromosome L. panicufota, which morphologically stands so close to
L. hieracioides, its diploid counterpart, as to suggest an autoploid origin.
Homology is quite lacking among the chromosomes of this species in the
hybrid with 8-chromosome L. gaillardioides,
although the chromosomes
of L. gaillardioides
and L. hieracioides,
on the other hand, are highly
homologous. This indicates that L. paniculata is not of autoploid origin
from L. hieracioides, for in that case one would expect at least the 16
L. panicufota chromosomes to pair among themselves in such a hybrid.
The hybrid with L. gaillardioides
was the only one obtained using L.
panicufota as a parent. These cytological results suggest that L. panicufota has had an independent origin, possibly through amphiploidy, with
one at least of its ancestors now extinct. Despite its morphological
similarity to L. hieracioides, which caused it to be overlooked previously
as a taxon, Layia panicufota is shown by the experimental evidence to
have been amply distinct both cytologically and genetically probably
for a long time.
The genetic criteria of relationships among the species of Layia as
determined by crossability, the relative ease of gene exchange, and
fertility of F x hybrids and relative vigor of F 2 hybrids (a measure of
survival potential), show that the genus is composed of 5 species complexes or cenospecies. Three of these are monotypic, the other 2 are
composed of 6 species each. Using the terminology of the experimentalist,
all 15 species of Layia are ecospecies, but 3 of them are also monotypic
cenospecies.
It is revealing to compare the three kinds of discontinuity in the genus.
The 6 species of the group each of which has 7 pairs of chromosomes are
also shown by their crossability to belong to one cenospecies, so here the
cytological and genetical data agree. Morphologically there are 3 groups
among the 6 species, each being set off from the others by a strong