Hybrid breakdown

Toby Bradshaw toby at milton.u.washington.edu
Mon Jun 1 12:56:02 EST 1992

I work on the genetics of growth and development in interspecific
(usually) hybrids of Populus.  By way of introduction, Populus
(poplar, cottonwood, aspen) hybrids are the fastest growing
trees in temperate latitudes.  Maximum yields run around
30 bone dry metric tons per hectare per year over a six year
rotation.  The high rate of growth and adaptability to
marginal farmland with minimal inputs of fertilizer and 
labor have attracted some support from the Biofuels
Feedstock Development program at the DOE, and from the
pulp/paper industry here in the Pacific Northwest.

The genus has 25-30 species worldwide, usually grouped into five
sections.  Interspecific hybridization is easy within sections,
and becomes progressively more difficult between species from
less related sections.  Hybrids between the Tacamahaca and Aigeros
sections are particularly good biomass producers.
We work primarily on hybrids between P. trichocarpa, the PNW native
black cottonwood, and P. deltoides, eastern cottonwood.  The F1
hybrids show typical heterosis for growth.  There is transgressive
variation for other traits (like leaf size), but most F1 traits
are intermediate between the two parental species.  Unlike some
interspecific hybrids in other systems (Drosophila, Nicotiana,
etc.), F1 hybrid Populus are fully male- and female-fertile.
The F2 (and, to a lesser extent, the BC1 to either parental
species) show severe "hybrid breakdown", with few or no
advanced generation ofspring growing nearly as well as even the
parental generation.  Hybrid breakdown is listed by Stebbins
as one postzygotic isolating mechanism, but appears to be
much less common than mechanisms that result in F1 sterility
or inviability.  The hand-waving explanations for hybrid
breakdown are "incongruity between genomes" or "recombinational 
disruption of co-adapted gene complexes".  It's possible
that one of these explanations could be true, but there have
been (to my knowledge) no direct tests of either.  Presumably
any mechanism leading to hybrid breakdown represents a
barrier to sustained gene flow between the species involved.
We have the opportunity to look in detail at the genetics of this
phenomenon using an RFLP map of the Populus genome we are
making (240 markers mapped so far).  Unfortunately, follow-
on experiments that involve breeding are a little hard to
envision because of the long generation interval in trees.

I'm looking for an herbaceous model for hybrid breakdown.  Those
of you who are plant taxonomists, breeders, or ecologists may be
familiar with some species pairs whose F1 hybrids are heterotic
but whose F2 offspring "break down".  Ideally, the model species will
have a short generation time, be monoecious and self-compatible,
propagate vegetatively, and have some interesting natural
history that might explain why they've become reproductively
isolated in the first place.  I'm not at all averse to using
crop plants/wild relatives.  Since I think that the necessary
genetics can be done quickly using RAPDs, previous genetic
characterization would be helpful but not essential.  I'm
familiar with the work of Clausen, Keck, and Hiesey, but not
with any later work on the species they examined.

Email replies are welcome, and I'll post a summary of private
replies if there is general interest in mechanisms of
reproductive isolation.

Toby Bradshaw
Biochem and Forest Resources
University of Washington

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