1. North Carolina State University, Raleigh, NC 27695-7614
Exploring novel tools for island conservation through the mating of wild and laboratory strains
Megan Serr, Caroline Leitschuh, Dona Kanavy, Brinda Bhaskar, Brooke Wolff, Rebecca Sears and John Godwin
Department of Biological Sciences, Center for Genetic Engineering and Society, and the W. M. Keck Center for Behavioral Biology
North Carolina State University, Raleigh, NC
Mus musculus, are significant and invasive pests, particularly on islands, where they have often evolved in the absence of natural predators. With a team of graduate students at North Carolina State University (NCSU), I am investigating the potential of using genetically modified mice as a tool for eradication. To inform this aim it is necessary to further understand mate choice among mice. Little is known, however, about the behavioral differences between wild mice and lab mice in terms of mating behavior, and reproductive output. Wild mice captured from the Farallon islands, off the coast of California near San Francisco, have been successfully bred and maintained at NCSU to gain insight into a wild strain. Currently the lab mice we have at NCSU are either standard C57/BL6 (B6) or males with a naturally occurring meiotic drive (tw2).
Wild and Lab Mice
Sry Mice and Breeding Experiments
Large Enclosures and Mate Choice
The Farallon mice
The Farallon islands are a group of islands located about 30 miles off of San Francisco, California1.
The only terrestrial mammals currently on the island are invasive house mice1,2.
The mice show annual cyclical variation of population sizes with peak density in late summer and early fall (figure 1)
The Farallons have the highest mouse densities ever recorded in natural habitats at over 1300/hectare (490/acre)3,4
Average litter size is 7.8 pups under laboratory conditions
Open field experiments show higher levels of anxiety-related behaviors compared to lab mice (figure 2)
Novel Tool
The current method to remove invasive house mice involves spreading toxicants and this is the approach to be used by the USFWS for the Farallon islands’1.
This method is not always successful, can have non-target impacts, and raises animal welfare concerns .
We are exploring an alternative, genetically-based technique to remove invasive mice from islands.
The proposed gene drive technology should create an all male population, which would lead island mouse populations to collapse. To further explain our project we have created a website
p=.026
p=.043
Figure 4 Average litter size of Faralon mice compared to MmF females bred with lab mice
Will wild females mate with tw2 males in a complex environment?
Standard large cages for laboratory mice are 0.129m2, which restricts movement and prevents the formation of social hierarchies.
To simulate more natural environments we have constructed five 2.97m2 boxes and added enrichment in the form of hiding places, running wheels, and rope for climbing (figure 4).
In these larger enclosures, we plan to track the mice with radio frequency identification and use microsatellite analysis to determine paternity.
This work is being done to further understand mate choice between lab and wild mice and to examine the potential of using genetically modified mice as a conservation tool.
The Sry mice
The Sry gene is the male gene/testis-determining factor in mammals.
The goal is to put the Sry gene into the tw2 construct.
By placing the Sry gene into the tw2 allele all offspring born should be phenotypically male
Males born with two copies of the tw2 should be sterile as well as those born XX with the tw2(sry) (figure 3)
Figure 1 Cyclical variation in population abundance on Southeast Farallon
Figure 2 Average velocity of MmF and B6 in the periphery versus center of the open field
Figure 3 Sry male crossed with a wild MmF female.
Figure 5 Larger enclosures with MmF and Tw2 mice.
Will wild females mate with lab males?
Five B6 males were paired with ten Farallon females (MmF) and all successfully bred
Nine litters were compared to previous MmF litters and litter size was significantly lower for B6 x MmF hybrids (5.7) compared to the average for either strain (Farallons=7.8, B6= 7.05) ( figure 4)
Six tw2 males were paired with six MmF females and four gave birth
The litter size for hybrid pups was (5.4) which was significantly lower than the Farallon litter size of (7.8) compared to the average for either strain (Farallons=7.8, Tw2 =7.5) (figure 4)
The litter size from matings between wild and lab strains may be smaller.
This suggests it would be important to assess litter size in hybrids and MmF females to see if this persists when we reduce the genetic contribution of the lab strain
Literature Cited
1. Farallon National Wildlife Refuge South Farallon Islands Invasive House Mouse Eradication Project(2013).Revised Draft Environmental Impact Statement.
2. Schoenherr, A., Feldmeth, R. & Emerson, M. (1999)Natural History Of The Islands Of California. (University of California Press).
3. Seamons, K. (2013)World’s most rodent-packed island has just 2 options. USA Today
4. Didion, J.(2012). Genetic Analysis of an invasive house mouse population on Southeast Farallon Island.
5. Silver, L.(1995) Mouse Genetics Concepts and Applications. Mouse Genet.
6. Lyon, M.F. (2003). Transmisison ratio distortion in mice. Annual Review of Genetics, 37,
7. Campbell, K., Becj, J., Eason, A., Godwin, J., Gould, F., Holmes, G., Madden, F., Ponder, J., Threadgill, D., Wegmann, A., & Bazter, G. (2015). The next generation of rodent eradications: Innovative technologies and tools to improve species specificity and increase their feasibility on islands. Biological Conservation, 11,
The laboratory mice
C57BL/6 (B6) laboratory mouse
Most widely-used research mouse
Inbred for many generations
Average litter size 7.05
Tw2 laboratory mouse
Naturally occurring meiotic drive ( t-haplotype)
Tw2 haplotype has an inheritance rate of over 90% (Kanavy et al. unpublished)
Is not embryonic lethal
Males who inherit two copies are sterile
Male heterozygotes will pass the gene drive on to the next generation at 90+% frequency.
All female mice have normal fertility and will transmit the gene drive6.
Acknowledgements
-The Threadgill Lab of Texas A&M University
-Gerry McChesney and Anne Morkill of the U.S. Fish and Wildlife Service
-Russell Bradley from Point Blue Conservation Science
-Karl Campbell of Island Conservation