Lecture 14 – Hybridization and Introgression
An overlooked effect of invasive species occurs at the level of the genome. When
species are introduced that are closely related to native species, they may hybridize
with the natives. These genetic crosses can have many different outcomes.
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(1) May not produce offspring that can survive or produce very unfit offspring:
this undesirable from the native standpoint as it represents lost reproductive
effort and diminished fitness.
(2) Hybrids can reproduce and viable offspring are created
(1) Prefer other hybrids (= a potentially novel third species)
(2) Backcross with either or both parents
(3) Introgression, repeated gene flow between populations whose
individuals hybridize, and where hybrids backcross to one or both
parent populations can reduce the genetic integrity of the parent
population. Overtime, repeated backcrossing and hybridization
moves DNA from one population into the other. Both populations can
lose their distinct identity but with invasive species, the gene flow
often is asymmetric, with the native population being subsumed.
The end result is a loss of native diversity through changes to the genome.
Examples
- North American tamarisk populations are composed of two separate species
and a variety of different hybrids. A hybrid species is the most dominant
genotype on the landscape in North America.
- Spartina cord grass – native to east coast of North America. Moved to
England where it hybridized with the native creating an infertile, clonally
reproducing third species. After a while, a spontaneous doubling of
chromosomes created a sexually reproducing fourth species which became a
super-invader in England, Europe, and elsewhere.
- Mallard ducks assimilate native anatid duck populations everywhere they
have been introduced. Gene flow is disproportionately from mallards to
native ducks which gradually lose their identity after repeatedly
backcrossing with various hybrid forms
- California salamanders hybridize with introduced barred salamanders
creating hybrid swarms. The hybrids are bigger and more aggressive than
either parent population and they now dominate wetlands in parts of the
central valley. Their larger size increases predation on co-occurring newts
and native frogs
- Phragmites – native genotypes of Phragmites australis have been virtually
eliminated by a European genotype (haplotype M) which is broadly tolerant
of many more habitats.