Appendix A. Model description and output from individual model runs.
Population values used in the model were a constant instantaneous mortality rate of 0.326 (equivalent to 95% mortality after 9 years), age at maturity (95% mature) of 2 years and an annual recruitment of 1000. Except for annual recruitment, these parameters correspond to those for the Campaspe carp population in Australia (Brown and Walker 2004). Males and females had similar mortality rates and ages of maturity in this population, so population parameters were not sex-specific in this model implementation. Under these conditions, average generation time was 4.04 years, and adult female population size was 796.
For the effectiveness comparisons, we specified that genetic constructs would have 8 copies at release (unless specifically stated otherwise). We assumed an intermediate level of density dependence (α = 1) as the default level in the sensitivity analyses. At this level, the relationship between offspring and parent numbers is slightly domed and the maximum number of offspring produced at the population replacement level (Ricker 1959). Further assumptions are: a constant environment, no leakage of the construct, no extra mortality due to integration of each copy, no effects of the construct on mating advantage or offspring viability, a constant annual stocking rate of 1 year-old carriers equal to 5% of average wild-type births prior to the release of carriers (assumed to be all males for the three gender distorting constructs) and no additional efforts to non-selectively remove the pests, i.e., both carriers and non-carriers were equally vulnerable.
Model outputs included the numbers of males, females and sterile individuals specified by copy number, as well as the number of individuals carrying different copy numbers over time. Representative model output is shown in Fig. A1, for a single simulation with default conditions and the daughterless construct released on genotypic male carriers so that both males and females carry the construct in the population. In the model, genetically modified (GM) males and females increase soon after release, corresponding to the decline in wild type males and females (Fig. A1, top-right panel). After seven generations, wild type numbers have declined by 75% while GM males and females have increased to comparable numbers. Wild type males declined to negligible levels after 12 generations wild type females by 17 generations. At this point, stocking of GM carriers is reduced and the total population declines to negligible numbers after 22 generations. The number of GM males increases rapidly to an asymptote tightly linked to the release of copy number 8 male carriers, and remains high until the stocking of GM carriers is reduced (Fig. A1 middle-right panel). In contrast, GM females, that arise only after a GM male carrier has mated with a wild type female, show a slower increase, have a preponderance of individuals with lower copy number and decline as the wild type females decline (Fig. A1 lower-right panel). When the copy number of stocked carriers is reduced to 2 (Fig. A1 left panels), there is a similar replacement of wild type individuals with GM individuals but enough wild type individuals remain to breed and maintain the population (albeit at a slightly lower level).
FIG. A1 Model outputs for a daughterless gene construct for stocked carriers having copy numbers of 2 (LHS) and 8 (RHS). Top, population trajectories of wild type males and females, number of GM carriers released, GM genetic males, GM genetic females, and the total population; Middle, numbers of GM male genetic fish carrying1-8 copies of the daughterless construct; Bottom, numbers of GM genetic female fish carrying1-8 copies of the daughterless construct. Note that in this option, all of the GM females are phenotypic males. The copy number key applies only to the middle and bottom panels. |
Under the same default conditions (other than as noted in the text), four of the eight methods led to extinction of all viable females within 25 generations (Fig. A2): female-specific sterility, female-specific lethality, “daughterless”, and inducible mortality. The other four methods depressed the number of viable females, at amounts ranging from 4% for under-dominance to 34% for the Trojan gene. All eight methods also substantially reduced the absolute number of wild-type females, to the extent that all but two led to fixation of the genetic modification (Fig. A2). Fixation occurred most rapidly for the Trojan gene, but fixation occurred even when we only stocked carriers of a neutral gene, indicating that the effect was at least in part the result of the aggressive stocking strategy. In the pleiotropic Trojan gene, underdominance and mutual incompatibility scenarios, the effect of the stocking was to replace the wild-type with the equally viable genetically modified females, resulting in a stable pest population comprised of a different genotype than the original.
FIG. A2. Population trajectories for the eight methods compared, under default conditions. Methods are defined in Table 1. |
FIG. A3. Population trajectories for different complementary pest removal strategies. Environmental effects on recruitment have been removed to facilitate comparisons. A. Effects of complementary harvesting (H) of adult individuals. No control action shown as a control.; H = Non-selective harvests at 50% of annual mean natural mortality at carrying capacity, only; FS = Female sterility at a stocking rate of 2.5%, all other parameters as in Figure 5; FS + H all = Female sterility plus non-selective harvesting at 50%; FS + H females only = Female sterility plus 50% harvest mortality of wild-type and GM females only; FS + H wild-type only = Female sterility plus 50% selective removal of male and female wild-type individuals. B. As above, but showing supplemental 50 and 100% (of natural mean mortality at carrying capacity) harvest/removal of age 1 juveniles, with and without female sterility option. |
LITERATURE CITED
Brown, P., and T. I. Walker. 2004. CARPSIM: stochastic simulation modeling of wild carp (Cyprinus carpio L.) population dynamics, with applications to pest control. Ecological Modeling 176: 83–97.
Burt, A. 2003. Site-specific selfish genes as tools for the control and genetic engineering of natural populations. Proceedings of the Biological Society of London 270:921–928.
Ricker, W. E. 1954. Stock and recruitment. Journal of Fisheries Research Board of Canada 11:559–623.
Schubert, D., B. Lechtenberg, A. Forsbach, M. Gils, S. Bahadur, and R. Schmidt. 2004. Silencing in Arabidopsis T-DNA transformants: the predominant role of a gene-specific RNA sensing mechanism versus position effects. Plant Cell 16:2561–2572.
Thresher, R. E. 2008. Autocidal technology for the control of invasive fish. Fisheries 33:114–121.