I need some head and sex

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Try out PMC Labs and tell us what you think. Learn More. At the turn of the 19 th century the first observations of a female-biased sex ratio in broods and populations of the head louse, Pediculus humanus capitishad been reported. A study by Buxton in on the sex ratio of lice on prisoners in Ceylon is still today the subject of reanalyses. This sex ratio distortion had been detected in ten different countries.

In the last sixty years no new data have been collected, especially on scalp infestations under economically and socially more developed conditions. Here we report a female bias of head lice in a survey of school children in Argentina. This bias is independent of the intensity of the pediculosis, which makes local mate competition highly unlikely as the source of the aberrant sex ratio; however, other possible adaptive mechanisms cannot be discounted.

These lice as well as lice from pupils in Britain were carrying several strains of the endosymbiotic bacterium Wolbachia pipientisone of the most wide spread intracellular sex ratio distorters. Similar Wolbachia strains are also present in the pig louse, Haematopinus suissuggesting that this endosymbiont might have a marked influence on the biology of the whole order. The presence of a related obligate nutritional bacterium in lice prevents the investigation of a causal link between sex ratio and endosymbionts.

Regardless of its origin, this sex ratio distortion in head lice that has been reported world wide, is stable over time and is a remarkable deviation from the stability of frequency-dependent selection of Fisher's sex ratio. A female bias first reported in is still present over a hundred years and a thousand generations later. The early but comprehensive literature on human head lice, Pediculus humanus capitisreveals a surprisingly homogeneous female bias of the sex ratio world wide [ 1 - 3 ].

The first reported observation of more female than male lice on human he reaches back to Harding in in the USA [ 1 ]. Reanalyses of data collected in are still employed to deduce life history traits of human head lice [ 478 ]. The female bias of natural populations on he is conserved in the sex ratio of individual broods of experimental infestations [ 267 ]. Many of the people sampled in these studies had been afflicted by poverty, confinement or war.

However, in the last sixty years no data have been collected, especially on more normal levels of infestations. Local and temporary deviations from equality of the operational sex ratio should be expected as natural fluctuations. We wanted to find out whether this deviation is also maintained over time. Roughly one hundred years after the original surveys, we determined the sex ratio of adult head lice on pupils from 6 different schools in Argentina.

This cohort is different from earlier studies in that it does no longer show any extremely high infestations due to an increase in general living standards. Nevertheless, a female bias is still preserved. Sex ratio distortions in arthropods frequently originate from cytoplasmic or extrachromosomal factors and parasites of the host and are most often associated with the endosymbiont Wolbachia pipientis.

Wolbachia has been shown to induce parthenogenesis in haplodiploid Hymenoptera and diplodiploid Collembola, feminisation in terrestrial isopods, and male killing in many insect species [ 9 - 13 ]. A new bacterium from the Bacteroidetes group Cytophaga-Flavobacterium-Bacteroides, CFB has been linked with feminisation and parthenogenesis in mites and insects [ 14 - 16 ]. A total of he were screened for adult lice along 6 schools exhibiting a prevalence of This translates into an overall female bias of There is no difference between boys and girls.

This is also the first survey of sex ratio in human head lice for South America.

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Applying the standard deviation of the data in Fig. Each of the six schools shows individually a female bias and replicate the bias of all the populations, Fig. There is no difference between urban and rural schools. These in female bias correspond with the data by Buxton and others [ 5 - 7 ]. This suggests that the female bias is both conserved in space and time. Sex ratio of human head lice of school children in Argentina.

The graph shows the distribution and regression of female and male lice per head with many data points overlapping. of male lice per female louse per head. The columns show the frequency with which on individual he female lice were associated with no males, less than one male per female, equality, and more than one male per female.

The boundaries of equality include one standard deviation SD on each side, the columns 'less than 1' and 'more than 1' are minus one SD. Female bias of head lice per school. An infested head might be considered as a metapopulation, a school as a population, all six schools replicate the female bias; there is no ificant difference between the schools, Chi square 2.

The sex ratio of human head lice is independent of the infestation level. N: of he; the horizontal line indicates equality. Pediculid lice are diploid amphimictic and exhibit an extra mitotic division following meiosis during spermatogenesis. Pediculus are sexually dimorphic with a small proportion of hermaphrodites. Siblings are not gregarious, males are abundant, both sexes eclose at similar times, mating starts within one day after eclosure or encounter, males are very promiscuous, females do not seem to store sperm, frequent copulation is a requirement for laying fertile eggs and both sexes show active migratory behaviour.

At the moment it is difficult to determine the sex of immature stages and the sex-specific mortality of immature stages directly. However, single-pair matings sometimes produce all of one sex. Differential mortality of nymphs does not for the unusual sex ratio under non-crowding conditions. The sex ratio does not change with increasing age of the mother. No evidence has been found that any environmental factor has any effect upon the sex ratio. Only in a single case under very crowded conditions, a male bias has been observed on some he by Buxton [ 717 ].

He attributes this to a severely reduced life span of females [ 7 ]. The shortened life expectancy of females is ascribed to copulation prior to full sclerotisation leading to injury and death [ 6 ]. A selective reduction in life expectancy of females under crowded condition has also been observed in other insect taxa, e. However, highly infested he with a maximum of 1, lice in a study by Roy and Ghosh still showed a female bias [ 5 ].

He argues that the transmission rate was likely to be low simply because the prison host population consisted of adult males only and because the prisoners couldn't get rid of their lice; these infrapopulations would subsist through several generations. This is neither supported by our data nor the data of Buxton, Roy and Ghosh, or Lang [ 5 - 7 ]. There is no evidence that female lice can manipulate the sex ratio of their offspring. Whoever has or has had school children to look after can confirm that individual he of pupils do not constitute a viscous population.

Exchange of adult lice between he is constantly ongoing and is a major reason for the continuous spread of lice in our society. Our data show that the female bias is present on individual he as well as in all six schools surveyed. All the schools are coeducational. Hamilton has proposed local mate competition as an explanation of female biased sex ratios in insect and mite populations.

All the arthropod populations listed by Hamilton are characterised by inbreeding and haplodiploidy [ 19 ]. An ideal for an extreme biofacies may be described by eight properties. One, the primary sex ratio is spanandrous — that is, females greatly preponderate. In human lice, the operational sex ratio is close to equality. The sex ratio determined by dissecting second instar nymphs and the sex ratio at eclosion of imagines of experimental broods is not more spanandrous than the operational sex ratio.

Broods of individual female lice exhibit the whole spectrum from male only offspring to female only offspring. Two, reproduction is arrhenotokous. Lice are diplodiploid and reproduction is amphimictic. Three, there is at least one male in every batch of offspring. Egg laying in the human head louse starts with the first blood meal regardless of insemination; it is more continuous given sufficient food than in batches.

The complete brood of a female might contain only males, only females or any ratio in between. Four, there is gregarious development, as a group of siblings, from egg to adult.

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Unlike the body louse where the female tends to return to the same spot for oviposition and which will lead to a clustering of eggs, siblings of head lice are practically not gregarious. Five, adult males eclose first and can mate many times. Both sexes of head lice eclose at similar times but males are very promiscuous. Six, mating takes place immediately after or even before eclosure of adult females. Mating continues to take place at any time of the day or night and during the whole adult life span of lice. Mating happens as much before as after dispersal and both sexes show the same active migratory behaviour.

Seven, males are disinclined, or unable, to emigrate from the batch. Male lice disperse as eagerly as females. This is very obvious in our data in the of he carrying male lice only Fig. Eight, females can store sperm; one insemination serves to fertilise the whole egg production. Female lice are stated to have no receptaculum seminiis spermatheca and therefore are considered not to be capable of storing sperm. Nevertheless, Bacot reported in a record observation for a female being able of laying fertile eggs twelve days after the removal of the male; however, the maximum ever observed by Nuttall in is only five days, which is in line with the assumption of limited or no sperm storage capability.

These eight reproductive properties of lice make local mate competition as the cause of a female-biased sex ratio completely impossible. Further work on Hamilton's hypothesis has shown that within each local group, even if founded by a single female, the sex ratio favoured by individual selection is equality [ 20 - 23 ]. However, groups founded by female-biased genotypes contribute more individuals to the population as a whole than groups founded by unbiased genotypes [ 24 ]. This requires the ability of kin recognition and selection. However, in the female-biased parasitoid wasp Nasonia vitripenniswhich is often used in textbooks as an example of local mate competition, mothers cannot discriminate between kin [ 25 ].

In many cases in which the female bias could not be attributed to local mate competition, local resource competition among females or female-biased dispersal have been put forward as alternative explanations [eg. Local resource competition and differential dispersal can be ruled out in head lice for the same reasons discussed earlier for local mate competition. Why, for example, parasitoid wasps like Melittobia digitata that seemingly fit all the requirements set out by Hamilton fail to meet the sex ratio prediction of the local mate competition hypothesis remains unsolved [ 28 ].

Orzack emphasises that the occurrence of local mate competition is not documented for most species whose sex ratios have been explained by it [ 29 ]. He also documents that the original evidence presented by Hamilton in support of local mate competition is not as clear as originally assumed. This suggests that major adaptive and parasitic sex ratio effecting mechanisms still remain to be discovered. Hamilton himself detected a maternally inherited microorganism, later identified as the endosymbiont Wolbachiain four species of parasitoid wasps belonging to the genus Trichogrammaa genus originally listed as an example of an extreme biofacies leading to local mate competition.

Removal of the bacterium changed reproduction from female only thelytoky to biparental arrhenotoky [ 30 ]. The possibility that maternally inherited bacteria might bias the sex ratio in favour of females has long been predicted [ 31 ]. Parthenogenesis-inducing Wolbachia are quite common in parasitoid wasp species [ 32 ]. However, in some species of female-biased parasitoid wasps Wolbachia does not influence the sex ratio directly. In double infected Nasonia for example, two strains of Wolbachia induce cytoplasmic incompatibility, in Asobara tabidaone of the three Wolbachia strains of the triple infection is essential for oocyte maturation, and in Melittobia australicathe effect of Wolbachia remains to be determined [ 33 - 35 ].

A sex ratio bias is often more likely related to the physiological state of its host and is consequently not directly controlled by the mother [ 36 ]. The physiological state includes the manipulation by symbiotic microorganisms. In the analysis and interpretation of sex ratio deviations, the discrimination between primary sex ratio and functional sex ratio is pivotal [ 37 ]. This allows the identification of symbiotic sex ratio distorters.

Using the polymerase chain reaction and specific primers for 16S rDNA and the Wolbachia outer surface protein gene wspwe demonstrated the presence of Wolbachia pipientis in two blood-sucking Anoplura species, the human louse Pediculus humanus capitisPediculidae, and the pig louse, Haematopinus suisHaematopinidae. Primers for insect-like CFB bacteria did not amplify any product; primers for mite-like CFB bacteria resulted only in unrelated lice sequences indicative for the absence of a specific target sequence.

The highest amounts of Wolbachia were detected in thorax and head. All lice tested of both species harboured Wolbachia.

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The Wolbachia infection is likely close to fixation, which suggests either a relatively young infection showing no s of host resistance or indicates an essential function for Wolbachia in the development and survival of its lice hosts. The fact that two phylogenetically very different lice species with two distinct hosts show similar high levels of penetrance of Wolbachia infection on two different continents le us to expect Wolbachia infections to be widespread in many lice species.

The wsp gene of Wolbachia is one of the fastest evolving. Wsp sequences of Wolbachia from human and animal lice overlap. No Wolbachia lineage specific to a louse species or to a louse host could be identified. Comparison of the wsp sequences revealed no special position of lice Wolbachia among other insect Wolbachia strains Fig.

All lice showed double infections with two different Wolbachia strains based on wsp sequence.

I need some head and sex

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