lost upon domestication (see Fig. 6.1, lower
graph). However, captive rearing based on
inbred lines has many practical difficulties.
Despite these difficulties, it may be particu-
larly advantageous when animals are
needed only at certain times a year or for
animals that are particularly difficult to
replace. This solution necessitates the main-
tenance of many inbred lines as the genetic
reservoir. Roush and Hopper (1995) advo-
cate 25–50 lines, although it is important to
note that a substantially larger number (
100) would be required both to have confi-
dence that even moderately rare alleles (P
0.2) would be retained and to provide a
buffer against the loss of some difficult-to-
maintain lines.
This method is obviously inappropriate
for species that are difficult or impossible to
maintain as inbred lines. Diploid animals
generally carry a significant load of deleteri-
ous recessive alleles and exhibit marked
inbreeding depression. This can make the
captive rearing of inbred lines difficult. In
haplodiploid animals, deleterious recessives
are much less of a problem; however, in
some groups of Hymenoptera problems are
caused by the production of diploid males
under inbreeding (see Luck et al., 1999). On
the other hand, the micro-Hymenoptera gen-
erally show negligible inbreeding depres-
sion, at least under laboratory conditions
(e.g. Sorati et al., 1996).
Inbred individuals are generally unsuit-
able for release because of their low fitness.
Even when this is not the case, the number of
distinct genotypes is limited to the number
of lines. Thus, prior to release, the inbred
lines should be crossed in some systematic
way to create a population of F 1 hybrids.
These F 1 hybrids (or better still their F 2 or F 3
offspring, creating recombinant genotypes)
can then be released.
Maintaining inbred lines is often imprac-
tical, but other methodologies can be used to
minimize detrimental adaptation to captive
rearing. One possibility is to design the rear-
ing facility to select for the maintenance of
specific traits. Boller (1972) suggested
adding ‘luxury’ stimuli, e.g. mating sites,
and using ‘suboptimal’ conditions, e.g. tem-
perature variation. Another approach is to
reduce selection for early reproductive
maturity by, for example, using older
females to lay eggs (see Saul and McCombs,
1995). In addition, it is possible to intermit-
tently select the population under more nat-
ural conditions. Mackauer (1976) suggested
that readaptation to field conditions could
be promoted by maintaining the colony for
one or more generations in field cages.
Another possibility, appropriate for natural
enemies, is to recapture some of the released
individuals and reintroduce them into the
colony. This strategy is likely to be very ben-
eficial, since recaptured individuals are a
selected group of genotypes able to survive
under field conditions.Colony augmentation or replacementA genetically variable captive population
will inevitably adapt to its new environment,
following a trajectory of the type shown in
Fig. 6.1. The relatively stable ‘domesticated
stock’ is the well-adapted end-point
(although long-term inbreeding will eventu-
ally lead to a deterioration in the popula-
tion). Unfortunately, this domesticated stock
is extremely unlikely to be at the optimum
that maximizes the effectiveness of a release
programme. In general, the optimum will be
either the non-adapted wild population
(lower graph, Fig. 6.1) or, probably more
usually, some intermediate between the wild
and domesticated forms (upper graph, Fig.
6.1). As a result, there is a strong argument
for either regularly augmenting the captive
population with genotypes from the wild or
replacing old populations with new ones
after a defined number of generations.
This practice of augmentation and/or
replacement is always necessary unless the
captive population is maintained as a large
number of inbred subpopulations. Roush and
Hopper (1995) suggest that a mixed strategy
may often be appropriate, with the inbred
lines providing a backup for a large colony.
Indeed, inbred lines can be used instead of
wild-caught individuals to regularly aug-
ment a large colony, as outlined below.
Soemori and Nakamori (1981) proposed
that melon-fly stocks should be replacedManaging Captive Populations 81