72
DAVID D. KECK
amphiploids as determined by morphological, cytological, and distributional evidence, as well as the early classical cases of experimentally
produced amphiploids, a number of which replicate wild species.
The number of amphiploids which by this time have been experimentally produced is very large, as may be gathered from the fact that
up to 1948 there were 69 authenticated cases known among the wheat
relatives alone (Sears, 1948).
One of the earliest syntheses of a wild species from its parents was
that made by Miintzing (1932), who reproduced the Eurasian mint,
Galeopsis tetrahit L., η = 16, from two European species, G. pubescens
Bess., η = 8, X G. speciosa Mill., η == 8. These three species are amply
distinct from one another, and they have different ranges and habitat
requirements. The F x generation between G. pubescens and G. speciosa
was diploid but quite highly sterile. Among the diploid F 2 plants that
were obtained was one almost sterile triploid. Upon backcrossing with
G. pubescens it produced the tetraploid (amphiploid) plant. The offspring of this were fertile, somewhat variable, but for the most part indistinguishable from natural G. tetrahit. A wild species had been synthesized anew in the laboratory, and an elementary evolutionary step had
been taken under controlled conditions.
There are now numerous bona fide cases with detailed histories recorded of natural species being artificially reproduced by amphiploidy:
Brassica napus L., η = 19, from B. campestris L., η — 10, χ Β. olerácea
L., η = 9 (U, 1935); Nicotiana tabacum L., η = 24, from N. silvestris
Speg. & Comes, η = 12, χ Ν. tomentosiformis
Goodsp., η == 12 (Kostoff,
1938); Madia citrigracilis Keck, η = 24, from M. gracilis (Sm.) Keck,
η = 16, χ Μ. citriodora Greene, η = 8 (Clausen et al. 1945); Gilia
clivorum (Jeps.) Grant, η — 18, from G. millefoliata F. & Μ., η = 9,
X G. achilleaefolia
Benth., η = 9 (V. Grant, 1954); Bromus
arizonicus
(Shear) Steb., η = 42, from B. haenkeanus (Presl) Kunth, η = 21, χ Β.
trinii Desv., η = 21 (Stebbins et al. 1944; Stebbins, 1949).
If this evidence of the way in which amphiploidy may operate in plant
evolution is in need of additional backing, one may cite the case of an
aggressive, thoroughly established species which has arisen through
this mechanism within historic times. This is Spartina townsendii H. & J.
Groves, η = 63, the cord grass of the tide marshes on the west coast of
Europe that combines the characters as well as the chromosomes of its
parents, the native S. stricta (Ait.), Roth, η = 28, and the introduced
North American species, S. alterniflora Lois., η = 35. S. townsendii has
proved to be so successful that it has nearly eliminated its parental
species wherever it has come in contact with them (Huskins, 1931).
DAVID D. KECK
amphiploids as determined by morphological, cytological, and distributional evidence, as well as the early classical cases of experimentally
produced amphiploids, a number of which replicate wild species.
The number of amphiploids which by this time have been experimentally produced is very large, as may be gathered from the fact that
up to 1948 there were 69 authenticated cases known among the wheat
relatives alone (Sears, 1948).
One of the earliest syntheses of a wild species from its parents was
that made by Miintzing (1932), who reproduced the Eurasian mint,
Galeopsis tetrahit L., η = 16, from two European species, G. pubescens
Bess., η = 8, X G. speciosa Mill., η == 8. These three species are amply
distinct from one another, and they have different ranges and habitat
requirements. The F x generation between G. pubescens and G. speciosa
was diploid but quite highly sterile. Among the diploid F 2 plants that
were obtained was one almost sterile triploid. Upon backcrossing with
G. pubescens it produced the tetraploid (amphiploid) plant. The offspring of this were fertile, somewhat variable, but for the most part indistinguishable from natural G. tetrahit. A wild species had been synthesized anew in the laboratory, and an elementary evolutionary step had
been taken under controlled conditions.
There are now numerous bona fide cases with detailed histories recorded of natural species being artificially reproduced by amphiploidy:
Brassica napus L., η = 19, from B. campestris L., η — 10, χ Β. olerácea
L., η = 9 (U, 1935); Nicotiana tabacum L., η = 24, from N. silvestris
Speg. & Comes, η = 12, χ Ν. tomentosiformis
Goodsp., η == 12 (Kostoff,
1938); Madia citrigracilis Keck, η = 24, from M. gracilis (Sm.) Keck,
η = 16, χ Μ. citriodora Greene, η = 8 (Clausen et al. 1945); Gilia
clivorum (Jeps.) Grant, η — 18, from G. millefoliata F. & Μ., η = 9,
X G. achilleaefolia
Benth., η = 9 (V. Grant, 1954); Bromus
arizonicus
(Shear) Steb., η = 42, from B. haenkeanus (Presl) Kunth, η = 21, χ Β.
trinii Desv., η = 21 (Stebbins et al. 1944; Stebbins, 1949).
If this evidence of the way in which amphiploidy may operate in plant
evolution is in need of additional backing, one may cite the case of an
aggressive, thoroughly established species which has arisen through
this mechanism within historic times. This is Spartina townsendii H. & J.
Groves, η = 63, the cord grass of the tide marshes on the west coast of
Europe that combines the characters as well as the chromosomes of its
parents, the native S. stricta (Ait.), Roth, η = 28, and the introduced
North American species, S. alterniflora Lois., η = 35. S. townsendii has
proved to be so successful that it has nearly eliminated its parental
species wherever it has come in contact with them (Huskins, 1931).