TRENDS IN SYSTEMATIC BOTANY
65
ticularly if the basic number of chromosomes is multiplied in some races
of the species (see under autoploidy below).
A very much rarer example of deviation in chromosome number within
a species is quoted from Clausen et al. (1937): "Two ecotypes of Hemizonia Parryi Greene were crossed, a smooth prostrate one from the
coast with 12 pairs of chromosomes [ssp. Congdonii (Rob. & Greenm.)
Keck], and a hairy erect one from the interior with 11 pairs of chromosomes [ssp. rudis (Greene) Keck]. In spite of this chromosome difference, the F x hybrid produced about 25,000 good akenes on 6 plants in
an isolation plot. A small fraction of these were sown and produced
about 2500 F 2 plants, all healthy and very vigorous, showing segregation
for the habit and pubescence characters. So far as we know, this is the
only established example of a chromosome difference between two
naturally occurring ecotypes of one species, i.e., where hybridization between two such forms did not produce partial sterility in F x and more
or less unbalanced and weakened offspring in F 2 ." It should be added
that many more chromosome counts made on members of these subspecies disclosed that an occasional plant with 12 chromosomes was to
be found in the predominantly 11-chromosomed ssp. rudis, and that 1
plant with 11 chromosomes was discovered in ssp. congdonii
which
otherwise had only 12 chromosomes. These subspecies are readily distinguished by several morphological characters.
The foregoing example, although very unusual, runs contrary to the
expected result of a difference in chromosome number signaling the
presence of a more or less strong sterility barrier. Together with the
autoploidy situation this case points to the fallibility of using the chromosome number alone as the sole criterion of species. This character must
be combined with others for intelligent use, although obviously it is one
of the strongest characters that can be employed.
Ordinarily a difference in chromosome number results in such a sterility barrier between groups. If the difference is the result of simple
doubling of the chromosome number (autoploidy), without subsequent
structural or mutational changes, it is obvious that the polyploid, although now separated from the "diploid" by a sterility barrier, will remain very similar to its ancestral form. In such cases the taxonomist must
content himself with recognizing the presence of a "cryptospecies" or
"genetic species" which cannot receive taxonomic recognition so long
as it lacks morphological definition. Fortunately, A. Love's proposal
(1951) that "all types differing in the number of chromosomes should
be classified as distinct species," and which he elaborated in a later paper
(Love, 1954a), has not gained wide acceptance as yet for such cases
65
ticularly if the basic number of chromosomes is multiplied in some races
of the species (see under autoploidy below).
A very much rarer example of deviation in chromosome number within
a species is quoted from Clausen et al. (1937): "Two ecotypes of Hemizonia Parryi Greene were crossed, a smooth prostrate one from the
coast with 12 pairs of chromosomes [ssp. Congdonii (Rob. & Greenm.)
Keck], and a hairy erect one from the interior with 11 pairs of chromosomes [ssp. rudis (Greene) Keck]. In spite of this chromosome difference, the F x hybrid produced about 25,000 good akenes on 6 plants in
an isolation plot. A small fraction of these were sown and produced
about 2500 F 2 plants, all healthy and very vigorous, showing segregation
for the habit and pubescence characters. So far as we know, this is the
only established example of a chromosome difference between two
naturally occurring ecotypes of one species, i.e., where hybridization between two such forms did not produce partial sterility in F x and more
or less unbalanced and weakened offspring in F 2 ." It should be added
that many more chromosome counts made on members of these subspecies disclosed that an occasional plant with 12 chromosomes was to
be found in the predominantly 11-chromosomed ssp. rudis, and that 1
plant with 11 chromosomes was discovered in ssp. congdonii
which
otherwise had only 12 chromosomes. These subspecies are readily distinguished by several morphological characters.
The foregoing example, although very unusual, runs contrary to the
expected result of a difference in chromosome number signaling the
presence of a more or less strong sterility barrier. Together with the
autoploidy situation this case points to the fallibility of using the chromosome number alone as the sole criterion of species. This character must
be combined with others for intelligent use, although obviously it is one
of the strongest characters that can be employed.
Ordinarily a difference in chromosome number results in such a sterility barrier between groups. If the difference is the result of simple
doubling of the chromosome number (autoploidy), without subsequent
structural or mutational changes, it is obvious that the polyploid, although now separated from the "diploid" by a sterility barrier, will remain very similar to its ancestral form. In such cases the taxonomist must
content himself with recognizing the presence of a "cryptospecies" or
"genetic species" which cannot receive taxonomic recognition so long
as it lacks morphological definition. Fortunately, A. Love's proposal
(1951) that "all types differing in the number of chromosomes should
be classified as distinct species," and which he elaborated in a later paper
(Love, 1954a), has not gained wide acceptance as yet for such cases