Table Of ContentRHODORA,
Vol. 99, No. 898, 152-160, 1997
pp.
NATURAL
HYBRIDIZATION THE GENUS
IN CIRSIUM:
ALTISSIMUM DISCOLOR—CYTOLOGICAL AND
X
C. C.
MORPHOLOGICAL
EVIDENCE
Simon Dabydeen
Biology Department, Frostburg State University,
MD
21532
Frostburg,
abstract. Cytological and morphological evidence
indicates natural hy-
bridization between Cirsium altissimum and
C. discolor. postulated that
It is
the high degree of hybrid sterility correlated with meiotic abnormalities.
is
Key
Words: Cirsium altissimum, Cirsium discolor, hybridization, hybrid
Nebraska.
sterility,
In the genus Cirsium (family Asteraceae, Cardueae),
tribe vari-
and make
ability intergradation of diagnostic characters morpho-
many
logical circumscription of species challenging and
difficult.
The
difficulty partly explained by the occasional breakdown of
is
between
sterility barriers closely related species, thus permitting
(Ownbey
interspecific hybridization Davidson
1951, 1964; 1963;
Bloom
Dabydeen
1977;
1987).
Cirsium
altissimum (L.) Spreng. and discolor (Muhl.)
C.
two
Spreng.
are biennial, sympatric species sharing similar mes-
ophytic habitats within an extensive eastern North American
range. Both species have a similar extended flowering ohenologv:
structures
immature
simultaneously on same
the plant. generally believed that
It is
members
of the genus Cirsium employ both autogamous and
al-
logamous
reproductive
strategies.
Petrak (1917) placed Cirsium altissimum and
C. discolor in his
A
Altissima
series to reflect close affinity. closer relationship
is
m
C
Oray
seen
s (1884) treatment of discolor as a variety of
altissimum Cnicus
(as altissimum).
C
Phenotypically, Cirsium
altissimum and
discolor are easily
and
distinguishable are separated primarily by the degree of lob-
ing or indentation of the leaves. According Frankton and
to
C
Moore
(1963), in discolor the "cauline leaves deeply
are pin-
C
narrow
natifid into linear-lanceolate lobes." and altissimum
in
152
—
Dabydeen
Cirsium
1997] Hybridization 153
the "cauline leaves are entire or subentire with shallowly
sinuate
lobes."
Cytologically, in Cirsium altissimum and discolor chromo-
C.
some
numbers have become
extremely
reliable in the determi-
nation of the species, even though supernumerary chromosomes
have been observed
infrequently in C. discolor (Frankton and
Moore Ownbey
Bloom
1963; and Hsi
1963; 1977).
Moore
Frankton and measured chromosome
(1963) somatic
=
=
lengths of Cirsium and
discolor (2n 20) C. altissimum (2n
and found no between
18) significant differences the chro-
total
matin
) lengths of these taxa nor in the size ranges of the individual
chromosomes. same
Apparently, both genotypes contain the
amount
They
of material. argued that the reduction in the chro-
mosome
number may
of altissimum have been accomplished
C.
chromosomes
chromosomes
Bloom
mentioned of
(1977) that his results initial studies (un-
C
showed
published) of Cirsium altissimum and discolor hybrids
chromosomal two by
that arrangement of these species differ a
minimum
of four reciprocal translocations and one paracentric
inversion.
Observation of a Cirsium plant with an unusual leaf lobing
from mixed
pattern (in Nebraska just across the Missouri River a
population of C. altissimum and C. discolor plants in Iowa)
prompted The mixed population was growing
this investigation.
in an area approximately by ten feet on top of the Missouri
thirty
River The was mostly with small
bank. vegetated,
area lightly
shrubs and Beyond riverbank patches of shrubs and
herbs. the tall
few what appeared be abandoned
a small dominated to
trees in
pasture
lands.
based on
In Cirsium altissimum and discolor, reports anal-
C.
yses of morphological have revealed evidence of natural
variation
Ownbey To
author's
hybridization (Davidson 1963; 1964). this
knowledge, no evidence supporting spontaneous hy-
cytological
bridization between altissimum and C. discolor exists in the
C.
chromosomal
of ev-
literature. Thus, paper the report
this is first
two
idence of natural hybridization between these taxa.
AND METHODS
MATERIALS
Cirsium
In July 1990, a plant with features similar to altissi-
and
•
154 Rhodora
99
[Vol.
riverbank
This
yards west of the Missouri River
linking Iowa.
This plant was one of a few plants left intact after a recent
was
brush-cut. observed that dried-up remnants of additional
It
Cirsium formed
plants part of the debris, but no other live Cir-
sium plants were observed in the vicinity of the putative hybrid
riverbank
mixed
C
C
ulation of nine altissimum and seven
discolor plants grew.
No
plants phenotypically from two were
different the species
C
found at this The occurrence of a mixed altissimum and
site.
proximity
brush-cul
and/or
their derivatives.
The
putative hybrid specimen was sampled
in addition to spec-
imens of the Iowa Cirsium altissimum and mixed
discolor
C.
Two
population. immature capitula and twenty- seven cypselae
long aborted ovaries
Seven immature
ant.
three
and immature
and
five capitula
thirtv-three
mature
mixture
V/V). Cypselae were germinated
chromosome
Microsporocyt
method
Chromosome
but firm
itle r
Chromosomes
lip.
microscope-earner
X
with 97
tern 5 ocular
Data for morphological comparison were from mea-
obtained
surements of nineteen characters (Table The mean values of
1).
measurements
10 for each character of each were
plant used.
ran
and
—
Dabydeen
1997] Cirsium Hybridization 155
Cirsium discolor: iowa, Freemont Co., riverbank, 50 yards west of
the
Missouri River
interstate bridge linking Nebraska 27
City, Jul
Dabydeen
1990, 9001, 9002, 9003
(neb).
Cirsium altissimum: iowa, Freemont Co., riverbank, 50 yards west of
the Missouri River interstate bridge linking Nebraska City, 27 Jul
Dabydeen
1990, 9004, 9005, 9006
(neb).
X
Cirsium
altissimum Cirsium Nebraska, Otoe
discolor: Co., roadside
riverbank, 75 yards west of the Missouri River interstate bridge
linking Iowa, 27 Jul 1990, Dabydeen 9007 (neb).
RESULTS
=
Cirsium altissimum has a chromosome number of 2n 18
(Figures la and chromosome complement
In C. discolor, the
If).
=
is 2n 20 (Figures lb and The suspected hybrid individual
lc).
has a complement of 19 chromosomes (Figure All chromo-
Id).
some
complements observed metaphase composed
mitotic are
at
of metacentric submetacentric chromosomes.
to
In most florets of the two immature capitula of the suspected
hybrid, microsporogenesis had been completed and only
already,
a few anthers with microsporocytes two stages of meiosis were
at
found. Meiotic analysis of the putative hybrid specimens revealed
that some of the bivalents were closely associated on an end-to-
end
basis and chain formation. In Figure a chain
illustrated 2g,
of six chromosomes, six bivalents, and one univalent (arrow) are
shown. Microsporogenesis was not normal; one of the
at least
four microspore appeared small (Figure
nuclei relatively le).
Only
three of twenty-seven (11%) cypselae of suspected hybrid
origin germinated; cypselae of both Cirsium altissimum and C.
100%
discolor had germination.
Measurements of nineteen characters (Table of the suspected
1)
com-
hybrid showed were intermediate
individual that fourteen
pared Three characters were approximately
to the parental types.
the same as either parental type, one character was larger, and
one
smaller than either of the parental types.
DISCUSSION
hybrid phenotypes often
In interspecific hybridization events,
many
express morphological intermediacy of parental characters.
In the suspected Cirsium hybrid specimen, fourteen characters,
including four leaf features, were intermediate between those of
Rhodora
156 99
[Vol.
a
Figure Photomicrographs of mitotic and meiotic chromosome comple-
1
.
X
ments of the genus Cirsium, 800 unless otherwise stated, C. altissimum:
a.
= =
mitotic metaphase, 2n 18. b. C. discolor: mitotic metaphase, 2n 20. c.
X
C. discolor microsporocyte: metaphase 10 bivalents. d. C. altissimum C.
I,
X
discolor, mitotic metaphase, 2n 19. C. altissimum C. discolor: ab-
e.
X
normal microsporogenesis and micronucleus (arrow) 300. C. altissimum
f.
metaphase 9
microsporocyte: bivalents.
I,
—
Dabydeen
1997] Cirsium Hybridization 157
<3
o
Q>
A
Figure 2. photomicrograph (X800) and an interpretive drawing of the
» meiotic chromosome complement of the genus Cirsium. C. altissimum X
g.
microsporocyte
arrow
formation
arrow
Table Comparison of character measurements of Cirsium altissimum,
1.
C. discolor, and their hybrid derivative (all measurements in millimeters).
altissimum
C.
X
C. altissimum C. discolor C. discolor
Mean Mean Mean
Characters S.E. S.E. S.E.
Involucre: length 29.7 0.48 28.2 0.85 26.8 1.01
width 21.6 0.85 22.3 0.85 23.7 1.03
Phyllaries: no. of rows 12.3 0.15 11.5 0.25 11.2 0.33
spine length 3.7 0.15 3.7 0.15 4.0 0.26
inner: length 26.1 0.53 24.3 0.47 22.4 0.70
inner: width 1.7 0.09 1.5 0.13 1.5 0.17
Pappus length 26.1 0.44 25.1 0.50 24.7 0.75
Corolla: lobe length 8.6 0.22 8.0 0.26 8.0 0.33
tube length 0.18 8.2 0.20 6.0 0.26
9.1
Stamen length 17.5 0.36 16.4 0.40 13.3 0.34
Stigma length 0.14 6.1 0.23 5.0 0.29
6.1
Style length 32.9 0.78 32.3 0.54 31.8 1.33
Leaf: length 100.8 3.78 103.4 1.96 112.5 3.57
0.62
width 18.4 1.30 16.4 0.62 15.4
0.90 22.0 0.77
lobe length 1.04 20.2
18.5
lobe base width 11.3 0.82 9.3 0.42 10.2 0.49
0.29
spine length 4.4 0.18 5.0 0.26 4.8
Intemode length 22.3 1.27 21.1 1.20 20.6 1.11
Flower length 39.0 0.84 39.0 1.62 36.0 1.33
Rhodora
158
99
[Vol.
Ownbey
C. altissimum and C. discolor (Table (1951) inves-
1).
tigated C. discolor, C. altissimum, and their putative hybrid de-
rivatives and determined that plants with intermediate leaf types
were
hybrids.
=
The diploid chromosome complement of 2n 18 in Cirsium
altissimum has been reported previously and number
the re-
is
Moore Ownbey
garded as stable (Frankton and 1963; and Hsi
Johnson chromosome num-
1963; 1974). In C. discolor, a diploid
=
ber of 20 has been determined (Frankton and Moore 1963;
2/i
Ownbey
Bloom
and Hsi
1963; 1977). In this taxon, accessory
chromosomes
have been
occasionally observed, but in a closely
C
related congener, muticum, frequent observations of extra chro-
mosomes
have been reported (Frankton and Moore
1963).
Accessory chromosomes do
usually not exert phenotypic influ-
ence. Thus, based on morphological data (Table and also cy-
1)
tological features (Figures Id and seems very
likely that
2g),
it
chromosomes
the nineteen observed in the putative hybrid spec-
imen
are products of interspecific hybridization between Cirsium
altissimum and These chromosomes,
C. discolor. nineteen there-
fore, likely do not constitute a genotype of a C. discolor specimen
with supernumerary chromosome.
a probable 10 chro-
that
It is
mosomes
must have come from C. discolor and 9 chromosomes
from
C. altissimum.
The
observation of a chain of six chromosomes, six bivalents,
and one
univalent (Figure 2g) during meiosis of suspected hybrid
microsporocytes suggested that 9 chromosomes of Cirsium altis-
C
simum
and 9 chromosomes of discolor are homologous or
homeologous, and
that reciprocal translocation involved in the
is
formation of the meiotic The must have
configuration. univalent
Similar
formation)
Bloom
other Cirsium taxa (Dabydeen
1987; 1977).
nation of micronuclei during microsporogenesis wa:
chromosomal
to deficiency from meiotic ab
Lie arising
imilar micronuclei formation observed microsporo
in
X
Cirsium flodmanii undulatum was
C. hybrids attrib
ging chromosomal (Dabvdeen 19871
materials
The manv
of
failure ovaries
may
There
external evidence of damaee would nrevent
that fruit
—
—
Dabydeen
Cirsium
1997] Hybridization 159
Many
formation. of the aborted ovaries were
shriveled or ex-
tremely The extremely low
flattened. rate of viability of cypselae
A
may
be due
to hybridity. high degree of of Cirsium
sterility
hybrids has been (Ownbey Bloom
reported
1951, 1964; 1977;
Dabydeen Bloom
1987). (1977) attributed the high degree of
ste-
irregularities arising
chromosomal
rearran
mixed
Cirsium discolor and C. altissimum not expected. Hybridization
is
events have occurred many where two
in instances these taxa
proximity
to
unknown
te,
and/or
It is suggested that the rather small size and possible recent
mixed
the absence of hybrids. At the Nebraska highly possible
site, is
it
may
that the brush-cut have destroyed a well-established mixed
population, including additional hybrids.
acknowledgments. The M.
author wishes thank K.
to
McCarty, R. C. Lommasson, Roland, and K. Planalp for their
J.
advice and
assistance.
LITERATURE CITED
W. Chromosomal between Cirsium discolor
L. 1977. differentiation
and
Bot. 1-13.
2:
Dabydeen, genus Cirsium: Cflodmanii
1987. Natural hybridization in the
S.
X
Rhodora 369-373.
undulatum.
C. 89:
Davidson, R. A. 1963. biometric survey of morphological variation
In—itial
222-241.
in Cirsium altissimum C. discolor complex. Brittonia 15:
Frankton, and Moore. Cytotaxonomy of Cirsium muticum,
C. R. 1963.
J.
Cirsium and Cirsium altissimum. Canad. Bot. 41: 73-84.
discolor, J.
—
Feulgen, R. and H. Rossenbach. 1924. Mikroscopisch chemischer Na-
vom Hoppe
chweis Typus Thymonucleinsaure.
Einer Nucleinsaure der
Seyl 203-248.
Z. 135:
Amer. Acad. Arts
Johnson, M. Cirsium, Carduus,
1974. Cynareae (Asteraceae) in Virginia.
F.
Onopordum. 152-160.
Virginia Sci. 25:
J. —
C
Ownbey,
genus Cirsium
G. B. 1951. Natural hybridization in the
I.
X muticum Michx. Torrey
discolor (Muhl. ex Willd.) Spreng. C. Bull.
Bot. Club 233-253.
78:
Rhodora
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160 [Vol.
— —
genus Cirsium altissimum
1964. Natural hybridization in the C.
II.
.
C
X Michigan 87-97.
Bot.
— discolor. 3:
and Chromosome numbers some North American
Y. Hsi. 1963. in
Rhodora 339-354.
species of the genus Cirsium. 65:
Petrak, Die Nordamerikanischen Arten der Gattung Cirsium. Beih.
1917.
F.
223-567.
Bot. Centralbl. 35:
Snow, R. 1963. Alcoholic hydrochloric acid carmine as a stain for chro-
mosomes 9-13.
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