Biology Reference
In-Depth Information
Compatibility and Breeding System
Catling (
1982
) tested taxa from northeastern North America for agamospermy,
autogamy, intrafloral selfing, geitonogamy, and xenogamy. He found autonomous
agamospermy (i.e., agamospermy without pollination) in widely dispersed popula-
tions of
S. casei
var
. casei
,
S. casei
var.
novaescotiae
, and
S. cernua
. Emasculated
specimens from these populations enclosed in insect-proof cages developed enlarged
ovaries containing abundant seed. Schmidt (
1987
) obtained similar results for
S.
cernua
, and Sheviak (
1982
) demonstrated normal capsule and seed development in
specimens of this species with excised columns.
Autonomous agamospermy was also observed in isolated populations of the oth-
erwise sexual species
S. magnicamporum
and
S. ochroleuca
located near the north-
eastern limits of their ranges. Agamospermous races of
S. magnicamporum
occurred
in extreme southwestern Ontario, eastern Michigan, northern Ohio, and part of
Indiana and Illinois, agamospermous races of
S. ochroleuca
in southern Nova Scotia
and western Prince Edward Island (Catling
1982
).
S. odorata
(
S. cernua
var.
odor-
ata
) was also agamospermic at 12 localities near the northeastern limit of its distri-
bution in New Jersey, Delaware, and northern Virginia (Catling
1982
).
The preceding six taxa are members of the
S. cernua
complex (e.g., Catling
1980b
). All are similar morphologically, bloom in the autumn, and were formerly
included in
S. cernua
(Correll
1978
; Catling
1982
).
S. odorata
,
S. ochroleuca
, and
S. magnicamporum
are diploid (2
n
= 30) (Sheviak
1982
).
S. cernua
is tetraploid; a
few triploids and aneuploids have also been found (Sheviak
1982
). Counts of chro-
mosome numbers for
S. casei
vary between 60 and 75 (Catling
1980b
).
Sheviak (
1982
) established a partial correlation between ploidy level, mode of
reproduction, and seed type in this complex. Diploids are sexual, undergo regular
meiosis with perfect bivalent formation, and produce monoembryonic seed.
Polyploids show variable levels of meiotic irregularity, as well as different levels of
agamospermy and polyembryonic seed production, extruded embryos, free embryos,
and ruptured embryo sacs.
Contrary to Swamy (
1948
), Sheviak (
1982
), among others, reported a high per-
centage of monoembryonic seed in the agamospermic allotetraploids of this com-
plex. Single embryos were usually present in 40-60% of mature seeds, but their
incidence ranged from 20 to 85% (Catling
1980b, 1982
). On the other hand, poly-
embryony is extremely rare or absent in obligately sexual individuals (Sheviak
1982
; Schmidt and Antlfinger
1992
). Based on these correlations and an examina-
tion of some of Catling's material, Sheviak (
1982
) concluded that the specimens of
S. magnicamporum, S. ochroleuca,
and
S. odorata
examined by Catling from near
their northeastern range limits were polyploid. However, Catling's (
1980b
) report of
a diploid, agamospermic specimen of
S. magnicamporum
, acknowledged by
Sheviak (
1982
), indicates that the correspondence is not perfect.
Based on direct anatomical studies of
S. casei
var.
casei, S. casei
var.
novaesco-
tiae
,
S. cernua
, and an agamospermous race of
S. magnicamporum
, Catling (
1982
)
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