Aby wyświetlić tę treść, wymagana jest subskrypcja JoVE. Zaloguj się lub rozpocznij bezpłatny okres próbny.
The present protocol describes how to measure common life parameter data in Aedes aegypti mosquitoes, including fecundity, wing size, fertility, sex ratio, viability, development times, male contribution, and adult longevity. These measurements can be used to assess the fitness of transgenic mosquitoes.
Transgenic mosquitoes often display fitness costs compared to their wild-type counterparts. In this regard, fitness cost studies involve collecting life parameter data from genetically modified mosquitoes and comparing them to mosquitoes lacking transgenes from the same genetic background. This manuscript illustrates how to measure common life history traits in the mosquito Aedes aegypti, including fecundity, wing size and shape, fertility, sex ratio, viability, development times, male contribution, and adult longevity. These parameters were chosen because they reflect reproductive success, are simple to measure, and are commonly reported in the literature. The representative results quantify fitness costs associated with either a gene knock-out or a single insertion of a gene drive element. Standardizing how life parameter data are collected is important because such data may be used to compare the health of transgenic mosquitoes generated across studies or to model the transgene fixation rate in a simulated wild-type mosquito population. Although this protocol is specific for transgenic Aedes aegypti, the protocol may also be used for other mosquito species or other experimental treatment conditions, with the caveat that certain biological contexts may require special adaptations.
Survival of the fittest is the Darwinian idea that individuals who harbor genes best adapted to their environment will pass those genes to subsequent generations1. This means that it is fitness that determines whether their genes will survive. This more than 150-year-old concept is perhaps the most significant determinant of engineering a successful gene drive in transgenic mosquitoes. Gene drives, or the super-Mendelian inheritance pattern of a selfish genetic element that allows it to spread through populations2, are being explored for genetic pest management3. In the context of vector control, this strategy aims to either replace wild-type (WT) arthropods with those resistant to pathogens (population modification) or eliminate them all together (population suppression)4. However, transgenic mosquitoes often exhibit fitness costs (also called genetic load) in comparison to their WT counterparts, which means that the transgene will be lost in populations that can outcompete them. Coupling transgenes with a gene drive system is therefore necessary to offset any fitness costs and push the transgene through the population at levels greater than expected from typical Mendelian inheritance4.
Laboratory studies across mosquito vector species have shown that transgenes often display fitness costs5,6,7. For example, Irvin et al. measured various life parameters in Aedes (Ae.) aegypti engineered to express enhanced green fluorescent protein (eGFP) or transposase genes under Drosophila actin 5C or synthetic 3XP3 promoters, and compared them to the wild-type Orlando strain (the same genetic background from which they were derived)5. Most notably, they found that all transgenic strains had significantly reduced reproductive fitness5. Dependent on the transgene, some Anopheles (An.) mosquitoes expressing transgenes that inhibited Plasmodium parasite development also exhibited fitness costs6. Specifically, An. stephensi expressing a bee venom phospholipase (PLA2) under constitutive or bloodmeal-inducible promoters laid significantly fewer eggs compared to controls6. Those authors also found that transgenic Anopheles expressing a different transgene, an SM1 dodecapeptide tetramer did not exhibit fitness costs, leading them to conclude that transgene-conferred fitness costs are dependent on, at least likely in part, the effect of the transgenic protein produced6. Indeed, fitness costs may be attributed to transgene products, positional effects, off-target effects, insertional mutagenesis, or inbreeding effects in laboratory-reared strains7. Gene drives must therefore be robust enough to offset these transgene-induced fitness costs while also avoiding the development of insertions and deletions (indels) that block the drive itself.
Developmental or reproductive fitness costs can be measured in laboratory or cage studies7, with a caveat being that unknown factors in the field may also have an impact. Nonetheless, controlled fitness studies are an important first step when planning or evaluating genetically modified mosquito releases, such as a gene drive program, to determine whether the transgenic line will persist over subsequent generations. For example, Hammond et al. evaluated An gambiae CRISPR/Cas9-based-gene drives meant to disrupt the genes necessary for female fertility8. Through cage studies maintained for at least 25 generations, the authors found that the mosquitoes incurred gene-drive resistant alleles that blocked CRISPR-targeted cleavage and restored female fertility8. Their modeling efforts suggested that fertility in females heterozygous for the gene drive had the most dramatic impacts on gene drive fixation in simulated conditions8. Along these same lines, Ae. aegypti (Higgs' White Eye strain, HWE) engineered to express autonomous gene drive cassettes under different promoters or at different intergenic loci exhibited different rates of gene drive fixation in simulated populations9. Using MGDrivE modeling10, combined with measured rates of indel formation, maternal effects, and laboratory-collected life parameter data, the authors found that mosquito fitness most strongly influenced the persistence of the gene drive in simulated conditions9. Fitness costs that are most likely to impair gene drive efficiency are attributable to somatic Cas9 (over) expression or from the gene that is targeted, particularly in heterozygotes, rather than the intrinsic drive itself11,12,13,14,15,16,17.
Given its significance, fitness is an important factor for the ability of a transgenic line to persist over subsequent generations, and it can be used as an indicator for any physiological effects associated with a transgene. For example, off-target effects may be associated with fitness costs. In this case, backcrossing a transgenic line for several generations is recommended. Furthermore, crossing the transgenic line with one that is reflective of a field population may also be necessary to investigate how well the transgenic population can compete in the real world. To quantify fitness costs so that they can be comparable, this manuscript provides simple protocols for measuring common life history traits in Ae. aegypti mosquitoes, with a particular emphasis on fitness costs associated with transgenes, so that such studies can be more easily reproduced. Protocols include fecundity, fertility, sex ratio, viability, development times, male contribution, and adult longevity. Measuring wing length and area was also chosen as a fitness measurement as it correlates with thorax length18,19 and body size measurements, which is directly linked to bloodmeal size, fecundity, and immunity20,21. Although there are many ways to assess fitness, these parameters were chosen because they reflect reproductive success, are simple to measure, and are commonly reported in the literature.
NOTE: This protocol is written for transgenic and wild-type Ae. aegypti lines that have been previously validated and established. For more information on generating transgenic Ae. aegypti, see Kistler et al.22 and Coates et al.23. All experiments outlined below were performed under standard Ae. aegypti rearing procedures. Mosquitoes were maintained at 28 °C with 75%-80% relative humidity and a 12 h light/12 h dark cycle. A minimum of 100 individual egg papers per mosquito stain is highly recommended for statistical purposes. Comprehensive fitness studies take approximately 3 months to complete (Figure 1).
1. Measuring fecundity in female mosquitoes
2. Measuring wing length, area, and centroid size
3. Assessing egg fertility
4. Determination of sex ratio in the pupae
5. Determination of larvae viability
6. Determination of larvae-to-pupae development time
7. Determination of male contribution
8. Determination of mosquito longevity
Following the above protocol, the fitness of two mosquito lines were evaluated: (1) CRISPR/Cas9-mediated knock out of the Ae. aegypti D7L1 (AAEL006424) salivary protein and (2) Ae. aegypti lines expressing autonomous CRISPR/Cas9-mediated gene drives9. In the case of the former, a homozygous D7L1 knock-out line was established by exploiting the non-homologous end-joining pathway (NHEJ) to generate the disruption after microinjecting embryos with sgRNAs specific to the D7L1
Ae. aegypti fitness studies are often performed in the laboratory to assess fitness costs associated with transgenic cargo (e.g., gene drive elements) or gene knock outs, as discussed in this manuscript; however, these studies may be performed for a variety of purposes—any that aim to evaluate the health of an Ae. aegypti group, such as Wolbachia-infected30,31, insecticide resistant32,
The authors have nothing to disclose.
The authors would like to thank Drs. Bill Reid and Alexander Franz from the University of Missouri for their support with this protocol. The authors would also like to thank Dr. Benjamin Krajacich from the NIH/NIAID for his support with the R analysis. This study was funded by the NIH, grant number R01 AI130085 (KEO), and the NIH/NIAID Division of Intramural Research Program AI001246 (EC).
Name | Company | Catalog Number | Comments |
1 oz. translucent plastic souffle cups | WebstaurantStore | 301100PC | |
2 oz. translucent polystyrene souffle cups | WebstaurantStore | 760P200N | |
3 mL plastic pipettors | Cornin | 357524 | |
50 mL conical tubes | Any brand | ||
64 oz. white double poly-coated paper food cup | WebstaurantStore | 999SOUP64WB | for mosquito enclosement |
65 mm lens | Canon | MP-E 65mm f/2.8 1-5x Macro Photo | Canon Macro Photo MP-E 65mm, 7D-65mm-1X; zoom=1, 200, 6.3, ISO=100; for photographing wings or egg papers, although other cameras are likely sufficient |
Aedes aegypti mosquitoes | BEI | multiple strains as eggs are available | |
Artifical membrane feeders | https://lillieglassblowers.com/ | Meduim membrane feeder, Custom made, 33mm | Chemglass also offers, but sizes are wrong for us. Ours are about 3 cm? |
ATP | MP Biomedical | ICN15026605 | Any good quality ATP, 10mM filter sterile aliquots at -20 |
Autoclave | for sterilizing water for hatching | ||
Canon EOS 7D camera | Canon | 3814B004 | for photographing wings or egg papers, although other cameras are likely sufficient |
defibrinated sheep blood | Colorado Serum Co. | 60 ml, every 2 weeks | https://colorado-serum-com.3dcartstores.com/sheep-defibrinated |
Dual Gooseneck Microscope Illuminator | Dolan Jenner Fiber-Lite 180 | 181-1 System | |
Ethanol | |||
Forceps | Dumont | 5SF | |
Gauze | omnisorb ii | 4" non-woven sponges | |
glass microscope slide | Fisher Scientific | 12-544-2 | |
Glass Petri dishes, 100 × 15 mm | VWR | 75845-546 | for anesthesizing/manipulating mosquitoes on ice |
Hogs' gut | Any Deli | we buy in bulk, split, wash and store in small aliquots of ~4X12" at -20 in 50 ml conical | |
Ice | |||
Ice bucket | |||
Kimwipe | Fisher Scientific | 06-666A | |
Leica GZ4 StereoZoom microscope | for screening transgenic mosquitoes (if applicable) | ||
Paintbrush | AIT synthetic brush | size 10-0 | for manipulating larvae/pupae (Amazon) |
Panty hose | Walmart | L'eggs Everyday | Women's Nylon Plus Knee Highs Sheer Toe, 16 pairs (plus fits the carton) |
Pencils | Any brand | ||
Plastic containers for 2° storage of cartons | Walmart | Sterilite 58 Qt Storage Box Clear Base White Lid Set of 8 | |
Plastic containers for growing larvae | Walmart | Sterilite 28 Qt. Storage Box Plastic, White, Set of 10 | |
Plastic containers for hatching larvae | Walmart | Sterilite 6 Qt. Storage Box Plastic, White | |
polypropylene clear deli containers | WebstaurantStore | 127DM12BULK | 12 oz, or 16 oz if needed for bigger (127RD16BULK) |
Rubber bands | Office Max | #100736/#909606 /#3777415 | 12", #64 and #10 |
Rubber stopper | VWR | 217-0515 | for mosquito enclosement |
Sugar source, such as sugar cubes or raisins | |||
Tetramin flake food | Tetramin | 16106 | |
tpsDig | Stony Brook Morphometrics | A free morphometric image-processing software distributed online available at https://www.sbmorphometrics.org/ | |
tpsUtil | Stony Brook Morphometrics | A free morphometric image-processing software distributed online available at https://www.sbmorphometrics.org/ | |
White organza fabric 8” × 8” | FabricWholesale.com | 4491676 | Joann Casa Collection Organza Fabric by Casa Collection |
Whitman Grade 1 Qualitative Filter paper | Whitman | 1001-824 | for egg papers. The white color makes it easier to see the black eggs. |
Zapytaj o uprawnienia na użycie tekstu lub obrazów z tego artykułu JoVE
Zapytaj o uprawnieniaThis article has been published
Video Coming Soon
Copyright © 2025 MyJoVE Corporation. Wszelkie prawa zastrzeżone