Dear John and All
> In unconnected posts Martin Damus offered the view that importing many
> beestrains was good for variability, and James Kilty stressed the
importance
> of breeding from our local bees. I'm sure they're both correct...
> Can anyone help me understand this from a GENETIC viewpoint please?
[snip]
Nice questions. Pros and cons of increasing the genetic diversity of the
bees in your apiary/region/country? I'll come back to this one.
A race? A. mellifera which has, in a defined geographical area, evolved
through selection over a very long time to the local conditions (including
beekeeping practices) to succeed. A strain? A line of bees of whatever
origin, usually deliberately selected over years rather than unconsciously
over historical time, to possess a suite of characters desired by the
beekeeper and to show them stably as long as mating is controlled. A new
strain is not just a cross between 2 existing ones but could arise from it
if repeatedly selected for uniformity.
Consequences of hybridisation? In the first generation, as with plants,
hybrid vigour. The individual gets an advantage in metabolic terms, so can
grow more quickly, or end up bigger, or do more of what it likes to do. The
reasons for this are rather technical and still scientifically
controversial, so I'll not go into them here. (Can if you like!) If you
start with two true-breeding strains and make an F1, the result can be
vigorous and relatively uniform. One full set of genes from each parent,
nothing complex. However, when that individual reproduces (at least if it
goes through a process called meiosis, as the queen does), variable mixes of
genes from the two original strains get dispensed into the gametes, and so
the offspring are variable and often do not perform nearly as well as the
F1. Meiosis, by the way, is where the chromosomes from the 2 different
parents meet up to swap segments before separating again in a way that
halves their number. This mixing of genes might cause a disruption to
traits relying on sets of selected genes working together - might gentleness
be in this category? From that variable population you might, with isolated
mating and careful selection over many generations, be able to pull out a
useful strain with desirable characters. However deliberate breeding like
this is hard enough with plants that stand still, let alone long-flying
promiscuous serial bigamists!
Consequences of inbreeding? Many organisms show a decline in vigour as
inbreeding proceeds. Why? We all carry defective versions of genes but
these are usually masked by a healthy version. With inbreeding, if taken
too far, more and more genes may have defective versions exposed as both
members of the pair (remembering that most organisms, drones excluded, have
two copies of each gene). However, by selecting for the traits you wish
within lines of different parentage within an apiary, inbreeding should be
no problem.
So, if James Kilty prefers to work with local strains he has the advantage
of starting with genetic material already adapted to local conditions and
just has to select for traits which he wishes to bring out in that kind of
genetic background. The big problem is that the populations you start with
are less likely to contain the full range of genetic variation found in the
honeybee so you may miss something, and you don't get the hybrid vigour
which can be found in inter-strain matings. Of course local bees may
already be heavily selected for something really useful due to past
epidemics and associated heavy losses - let's speculate tracheal mite
resistance - and exotic material may not have this.
Martin Dumas liked variability. That can be good too - increasing hybrid
vigour and providing more genes in the genepool from which the perfect bee
may emerge, at the expense of a degree of unpredictability. So yes, you
were right - both are correct!
Hope that this explanation from the perspective of a plant geneticist made
some sense, and apologies to those who didn't want a genetics lesson!
best wishes
Gavin.
|
