Is Multifactorial Sex Determination in the House Fly, Musca domestica (L.), Stable Over Time?

Sex determination pathways evolve rapidly, usually because of turnover of master regulatory genes at the top of the developmental pathway. Polygenic sex determination is expected to be a transient state between ancestral and derived conditions. However, polygenic sex determination has been observed in numerous animal species, including the house fly, Musca domestica House fly males carry a male-determining factor (M) that can be located on any chromosome, and an individual male may have multiple M factors. Females lack M and/or have a dominant allele of the Md-tra gene (Md-tra D ) that acts as a female-determining locus even in the presence of multiple copies of M. We found the frequency and linkage of M in house flies collected in Chino, CA (USA) was relatively unchanged between 1982 and 2014. The frequency of females with Md-tra D in the 2014 collection was 33.6% (n = 140). Analysis of these results, plus previously published data, revealed a strong correlation between the frequencies of Md-tra D and multiple M males, and we find that these populations are expected to have balanced sex ratios. We also find that fitness values that allow for the invasion and maintenance of multiple sex determining loci suggest that sexually antagonistic selection could be responsible for maintaining polygenic sex determination in house fly populations. The stability over time and equilibrium frequencies within populations suggest the house fly polygenic sex determination system is not in transition, and provide guidance for future investigations on the factors responsible for the polymorphism.

Mechanisms of pyrethroid resistance in the dengue mosquito vector, Aedes aegypti: target site insensitivity, penetration, and metabolism.

Aedes aegypti is the major vector of yellow and dengue fevers. After 10 generations of adult selection, an A. aegypti strain (SP) developed 1650-fold resistance to permethrin, which is one of the most widely used pyrethroid insecticides for mosquito control. SP larvae also developed 8790-fold resistance following selection of the adults. Prior to the selections, the frequencies of V1016G and F1534C mutations in domains II and III, respectively, of voltage-sensitive sodium channel (Vssc, the target site of pyrethroid insecticide) were 0.44 and 0.56, respectively. In contrast, only G1016 alleles were present after two permethrin selections, indicating that G1016 can more contribute to the insensitivity of Vssc than C1534. In vivo metabolism studies showed that the SP strain excreted permethrin metabolites more rapidly than a susceptible SMK strain. Pretreatment with piperonyl butoxide caused strong inhibition of excretion of permethrin metabolites, suggesting that cytochrome P450 monooxygenases (P450s) play an important role in resistance development. In vitro metabolism studies also indicated an association of P450s with resistance. Microarray analysis showed that multiple P450 genes were over expressed during the larval and adult stages in the SP strain. Following quantitative real time PCR, we focused on two P450 isoforms, CYP9M6 and CYP6BB2. Transcription levels of these P450s were well correlated with the rate of permethrin excretion and they were certainly capable of detoxifying permethrin to 4'-HO-permethrin. Over expression of CYP9M6 was partially due to gene amplification. There was no significant difference in the rate of permethrin reduction from cuticle between SP and SMK strains.

Insecticide resistance in potential vector mosquitoes for West Nile virus in Japan.

Culex pipiens complex is the significant vector mosquito of West Nile virus. To take stock of the current situation of insecticide susceptibilities and design an ideal mosquito control strategy, we collected Culex pipiens pallens Coquillet, Culex pipiens form molestus Forskal, and Culex quinquefasciatus Say from fields in Japan and conducted bioassays for five larvicides (fenitrothion, temephos, etofenprox, diflubenzuron, and pyriproxyfen) by using a larval dipping method. Among five insecticides tested, obvious reduced susceptibilities were observed for etofenprox, which is the only pyrethroid compound registered as a larvicide in Japan. Twenty-two of 56 colonies exhibited a >10% survival rate at the etofenprox concentration of 5.7 microg/ml, which is a 10 times higher concentration of the working solution. The LC50 of a colony collected from Fukuoka prefecture for etofenprox exceeded 60 microg/ml (resistance ratio >2,307), and this colony also exhibited cross-resistance to other pyrethroids, permethrin (299-fold) and phenothrin (1,200-fold). The insect growth regulators diflubenzuron and pyriproxyfen were found to be sufficiently effective enough to control Culex larvae present, but decreased sensitivities to these insecticides were slightly detected in some colonies of Cx. p. form molestus collected from urban areas. Several etofenprox-resistant colonies of Cx. p. form molestus exhibited simultaneously decreased susceptibilities to other insecticides, including temephos, diflubenzuron, and pyriproxyfen.

A cline in frequency of autosomal males is not associated with insecticide resistance in house fly (Diptera: Muscidae).

Geographic variation in the chromosomal location of the male sex determining factor (M) was studied in four house fly, Musca domestica L., populations from the eastern United States. We found a strong clinal trend (29 degrees 41' latitude in Florida to 44 degrees 2' in Maine) in which the percentage of standard XY(M) males increased with increasing latitude. In Florida, 100% of the males possessed the M factor on the third autosome (III(M)). North Carolina had 20% III(M) males and 2.35% with both Y(M) and III(M). Fewer III(M) males were found in New York (4.35%). Populations from Maine contained 100% XY(M) males. In two of three standard laboratory-susceptible strains, all males carried M on an autosome ("autosomal males" or A(M)): CS (III(M)) and SRS (V(M)). Insecticide bioassays of four field-collected strains led us to conclude that resistance is not correlated with sex determination over a broad range of insecticides. For example, high levels of resistance to permethrin (86-99% survival at a diagnostic concentration) were found in all four field-collected strains. The five other insecticides evaluated showed varying levels of resistance among field strains. We conclude that a cline is present in house fly populations from the eastern United States with 100% III(M) males in the south and entirely Y(M) males in the north and that insecticide resistance is not a key factor influencing the evolution or linkage of M.

Linkage analysis of an acetylcholinesterase gene in the house fly Musca domestica (Diptera: Muscidae).

Linkage of an acetylcholinesterase (AChE) gene was detected in the house fly, Musca domestica L., by using the backcross method between a strain, aabys, that had a morphological multichromosomal marker on each of the five autosomes and a wild strain, LPR. Both strains were homozygous in this gene, and we used eight single nucleotide polymorphisms (SNPs) between them to distinguish the parental sequences in the backcrossed progeny, two of which resulted in the amino acid substitiutions common to the Drosophila and Aedes AChEs insensitive to organophosphates and carbamates. F, appeared to be a wild phenotype, and the AChE gene was heterozyous of aabys and LPR. In the backcross progeny, 32 (2(5)) phenotypes appeared, and 10 phenotypes with one wild or morphological marker were picked up for genotyping by the SNPs of AChE gene. A combination of the morphological markers and the SNPs revealed that the AChE structural gene is linked to autosome 2 in the house fly.