Biology Reference
In-Depth Information
Table 19.1.
Mean survivorship and root biomass of puzzle sunflower
(
Helianthus paradoxus
) and its parent species,
H. annuus
and
H. petio-
laris
, under irrigation by water of different salinities. (Data from Welch
and Rieseberg 2002.)
Helianthus
Helianthus
Helianthus
Salinity*
annuus
paradoxus
petiolaris
Survivorship (%)
0
100
100
100
100
75
100
33
200
42
83
8
Root Biomass (g)
0
2.3
1.2
0.4
100
0.4
0.7
0.05
200
0.05
0.2
0.003
*mmol/L NaCl
parents (table 19.1). Field studies with puzzle sunflower and both parents
in a saline habitat in New Mexico showed that natural selection favored
leaf succulence, high calcium uptake, and reduced uptake of sodium and
related cations (Lexer et al. 2003a). Laboratory analysis of the genetic basis
of these traits, and their relation to survivorship, showed that the hybrid
puzzle sunflower has achieved a recombination of traits fixed for opposite
influences in its parents (Lexer et al. 2003b).
Another example of speciation resulting from an alien introduction
involves the white cattail (
Typha glauca
), which appears to be a stabilized
hybrid between the native North American broadleaf cattail (
T. latifolia
)
and the introduced narrowleaf cattail (
T. angustifolia
) (Kuehn et al. 1999).
R. J. Abbott (1992) investigated the origin and spread of the Oxford
ragwort (
Senecio squalidus
). This species apparently originated by homo-
ploid hybridization on Mt. Etna, Italy. It occurs in an altitudinal range of
1,000-1,800 m, which lies between the elevations of its two parental
species. Introduced to Oxford, England, in the 1800s, isolated from its
parents, it spread along the expanding railway network and has become
widely established as an alien of homoploid hybrid origin (Abbott 1992).
Following its spread in Britain, Oxford ragwort has interbred with
common groundsel (
Senecio vulgaris
), with remarkable evolutionary results
(Abbott 1992). Oxford ragwort, with a diploid chromosome number of
20, and common groundsel, with a diploid number of 40, have
hybridized, yielding offspring with a diploid chromosome number of 30.
This form has backcrossed to common groundsel, giving rise to what is
