Engineering the genomes of wild insect populations: challenges, and opportunities provided by synthetic Medea selfish genetic elements

Abstract

Advances in insect transgenesis and our knowledge of insect physiology and genomics are making it possible to create transgenic populations of beneficial or pest insects that express novel traits. There are contexts in which we may want the transgenes responsible for these traits to spread so that all individuals within a wild population carry them, a process known as population replacement. Transgenes of interest are unlikely to confer an overall fitness benefit on those who carry them. Therefore, an essential component of any population replacement strategy is the presence of a drive mechanism that will ensure the spread of linked transgenes. We discuss contexts in which population replacement might be desirable and the requirements a drive system must satisfy to be both effective and safe. We then describe the creation of synthetic Medea elements, the first selfish genetic elements synthesized de novo, with the capability of driving population replacement, in this case in Drosophila. The strategy used to create Drosophila Medea is applicable to a number of other insect species and the Medea system satisfies key requirements for scientific and social acceptance. Finally, we highlight several challenges to implementing population replacement in the wild.

Figure 1

Medea genetics, and a possible mechanism by which Tribolium Medea acts. (A) When Medea is present in a female, only progeny that inherit Medea from one or both parents survive. (B) The genetics of Tribolium Medea, in particular the isolation of mutants that have zygotic rescuing, but not maternal killing activity, suggest a model in which Medea consists of two tightly linked genes, a maternally-expressed gene (Maternal toxin) whose product (red circles) causes developmental arrest of all eggs, and a zygotically expressed gene (zygotic antidote) whose product (green background) is able to rescue the normal development of eggs that inherit the element from either parent.

Figure 2

Population genetic behavior of Medea. Plot describing the number of generations required for Medea to be present in >99% of individuals, for a Medea element with different levels of a multiplicative embryonic fitness cost. Homozygous Medea male:non-Medea male introduction ratios are indicated on the Y axis, and embryonic fitness cost on the X axis. Area between lines indicates regions of parameter space within which a specific number of generations (indicated by numbers and arrows) are required for the frequency of Medea individuals to reach a frequency of 99% or greater. Line color, shown in the heat map at right, provides a rough measure of how many generations are required. Black lines (50+) indicate that fifty or more generations are required. The border between the black-lined region and the lower unlined region defines the critical Medea:non-Medea male introduction ratio (CMIR), below which Medea will be eliminated from the population. The model assumes an infinite population size, discrete, non-overlapping generations, and random mating.

Figure 3

Synthetic Medea elements result from zygotic rescue of a maternal loss-of-function that results in embryonic arrest. During wildtype oogenesis (left-side boxes, upper and lower) a maternal transcript is synthesized (green line). This transcript is translated during oogenesis, but the product is not utilized until early embryogenesis. When a female is heterozygous for Medea (red triangle; right-side boxes, upper and lower) a transgene drives maternal germline-specific expression of microRNAs that silence expression of the gene whose product is required for early embryogenesis. This results in inheritance of a lethal condition - the loss of an essential maternally deposited product - by all oocytes/embryos. Progeny survive the embryonic arrest thereby induced if they inherit from their mother (in this example) a tightly linked transgene driving early zygotic expression of the maternally silenced gene just in time to restore embryo development (box in lower right), but they die if they fail to inherit it (large red X in box in upper right). Circles indicate adult females (left side of each box) and embryos (right side of each box). Black lines in these circles represent a pair of homologous chromosomes. Medea is indicated by a red triangle and the chromosome inherited by progeny by a red asterix.

Figure 4

Chromosome breakage and rejoining can create Medea^Δeff and Medea^ins chromosomes, and can be prevented by placing the toxin and the effectors in an intron of the antidote. (A) Chromosome breakage and rejoining (illustrated by the dotted lines and the salmon-colored thick line) that separates the Medea element from its cargo results in the creation of a Medea^Δeff element, which lacks cargo. (B) Placing the cargo between the toxin and antidote genes prevents breakage and rejoining from creating a Medea^Δeff element, but it does not prevent the appearance of Medea^ins, an antidote-only element. (C) Splitting the rescue molecule into two, individually non-functional parts creates an element in which DNA breakage and rejoining events results in loss of rescue activity. The chromosomes thereby created cannot show maternal-effect selfish behavior, nor will they block the spread of intact Medea elements through the creation of rescue-only alleles.

Figure 5

Second generation Medea elements (Medeaⁿ⁺¹), comprised of a new toxin, a new antidote, a new cargo and the first generation antidote, can drive first generation elements (Medeaⁿ) out of the population at the same time as they are driven in, if both elements are located at the same position in the genome.