Any geneticdifferences that might be uncovered between populations or species must be ofprofound importance because of the low within-population component ofvariability (Avise, 2004).Although Muller (1950) predicted that only one locus in 1,000 (0.1%) wouldprove to be heterozygous in a typical individual (in Avise, 2004), proportions of polymorphic lociin a population range generally from 10 to 20% in vertebrates and from 25 to50% in invertebrates (Selander& Kaufmen, 1973).
The mean heterozygosity that found for 24 insectspecies was 0.155, for another 24 invertebrates species was 0.1507 and for 22 Vertebratesspecies was 0.0584 (Lester& Selander, 1979; Selander& Kaufmen, 1973). In the same matter, 23 Hymenoptera species(haplodiploidy system) showed the lowest mean value- 0.037. Evaluation of the probability of fixation of amutant gene in a diploid and a haplodiploid population led to the conclusionthat if the populations have equal numbers of genes, the values of fixation probabilityare approximately equal, but if the populations have equal numbers ofindividuals, the fixation probability in the haplodiploid population may reacha maximum of 1.
33 times that in the diploid population (Lester & Selander,1979). A largepart of the genetic variability in natural populations may be due to balancedpolymorphism. For sex-linked loci in the haplodiploid species, there are fivegenotypes to consider instead of the familiar three. Following LI (1967), thegenotypes for a pair of alleles are: AA Aa aa A a (Crozier, 1970).
InbreedingBreedingsystems in plants and animals vary, and may occur at any point on a continuumbetween the two extremes of self-fertilization (an extreme case of inbreeding)and outbreeding (Gottlieb etal., 2009). The effect of inbreeding on fitness is a major theoreticaland empirical issue in evolutionary and conservation biology (Pray & Goodnight, 1995).There are two classes ofeffects, which are ascribed to inbreeding: 1) a decline in all elements ofvigor (e.
g. weight, fertility, vitality) and 2) an increase in uniformitywithin the inbred stock, correlated with an increase in prepotency in outsidecrosses (Wright,1922). Inbreeding events in a normally outbredpopulation can lead to elevated levels of homozygosity, which may lead toinbreeding depression (i.e., a reduction in fitness of inbred offspringrelative to progeny from unrelated parents) that is largely due to theexpression of deleterious recessive alleles (Henter, 2003). In some highly inbred animals ithas been shown that there is a preference to produce a precise sex ratio thanto produce a binomial sex ratio, although the appearance of a precise sex ratiomay change as a result of cannibalism or combat among males (Green et al.
, 1982). The coefficient of inbreeding (F)represent the probability that two alleles at a locus in an individual areidentical by descent, i.e., that an individual is homozygous for an ancestralallele by inheritance and not by mutation (Rédei, 2008). The level of inbreeding depression in apopulation is an essential parameter in models of plant and animal mating systemevolution. If increases (in the level of inbreeding), as measured by Wright’sinbreeding coefficient F, exceed 1% per generation, then a population is atrisk for serious inbreeding depression. However, inbreeding depression may notbe a constant parameter that is identical for all populations of all species.Indeed, even Darwin (1876) reported inbred lines that actually increased infitness relative to an outbred control (Pray & Goodnight, 1995).
HaplodiploidyInvarious insects, the differentiation between the sexes is a matter of quantityof genetic material: females are diploid, while males are haploid. Fertilizedeggs yielded a diploid daughter and males develop fromunfertilized eggs.Therefore, haplodiploids may suffer less from inbreeding depression due to recessivedeleterious alleles in the genetic load that are hidden in heterozygous females,which will be expressed and thus purged through the hemizygote males (Henter, 2003). In highlyinbred, haplodiploidy parasitic wasps, mated females may produce most femalesand exactly one male. In other cases, they may produce larger broods with the number ofmales having lower than binomial variance. Therefore,mated females are able tocontrol their population sex ratio and balance the product of the unmatedfemales that produce males only. Natural selection should favor the productionof the number of males that maximizes the mean number of inseminated femalesemerging from each brood (Greenet al.
, 1982). For polymorphism, although inthe haplodiploidy case values have to be correct for five fitnesses for polymorphism(AA Aa aa A a), it seems that there is nosignificant difference between the numbers of polymorphisms possible at five orthree genotypes (Crozier, 1970). In contrast, haplodiploidy reducesrecombination and dominance effects and causes temporal oscillatory changes ingene frequencies between the genders in the populations of a specie. Moreover inthis system, linkage between genes and other loci should be increased becausethere is no opportunity for crossing over or chromosome segregation in males (Lester & Selander,1979). Increase thoselinkages may lead to a decrease individual’s heterozygosity, and later on it mayaffect the heterozygosity of whole population. One way to assessheterozygosity is to use DNA microsatellites(as an example of genetic markers) that are powerful tools for studying matingsystems, populations’ structures and for measuring paternity also for thediploid females in extreme inbreeders haplodiploidyspecies. The high variability, neutrality, codominance, and Mendelianinheritance of these markers make them well suited for investigating featuresof population genetics. They can be used to reconstruct pedigrees, calculaterates of inbreeding for natural populations, estimate parentage, and reliablyinfer demographic gene flow (Ráczet al.
, 2015; Berget al., 2003; Borquez & Brante, 2017). CoccotrypesdactyliperdaWithin thebark beetle family Scolytinae, regular inbreeding by sib-mating is one of themost successful ecological strategies. With currently 119 species assigned toit, Coccotrypes are found breeding in an exceptional variety of non-standardwoody tissues other than bark and phloem (Jordal et al.
, 2002). The date beetle, C.dactyliperda (Coleoptera: Scolytidae), is well known throughout the world,and in Israel it is a major pest of green, unripe date fruit (Blumberg & Kehat, 1982).
It breeds in the seeds of various palms; especially those of the genus Phoenixand are counted as one of approximately 1,400 described species in the Xyleborinitribe of predispersal, sib-mating beetles (Holzman et al., 2009). The sex determination mechanism of this stronglyinbreeding species is haplodiploidy and can be further classified as’Arrhenotokous’: where the haploid dwarf males are produced from unfertilizedeggs and they may mate with their diploid mothers or sisters, prior todispersal. The offspring sex ratio of matedfemales is strongly female-biased (Zchori-Fein et al., 2006; Holzman et al.
, 2009). Hence,unmated females are able to reproduce, although they will produce only maleoffspring. This in turn might lead to a reduction in their offspring’s successin encountering mating opportunities, thus incurring a reproductive cost (Gottlieb et al., 2014). Theoreticaland empirical studies indicated a selective advantage to mated females inhighly inbred haplodiploid populations where females can control precisely the genderof their offspring.
However, the occurrence of a high frequency of unmatedfemales in structured populations is not unusual for haplodiploids (Green et al., 1982;Gottlieb et al., 2014). C. dactyliperda reproductionrevolves around the date fruit’s stone. The adult female chews a roundbore-hole (1 mm diam.) in the fruit (Fig.
1), causing it to drop 1 or 2 dayslater. The dwarf males are unable to penetrate the stones. Eggs are laid ingalleries inside the date stone 1-2 days after the female penetrated the stone (Blumberg & Kehat, 1982).Males have reduced and non-functional flight-wings, and are rarely found alonein a seed (Gottlieb etal., 2014). A newly emerged unmated female colonizes a new datestone and the entire life cycle takes place inside that seed (Zchori-Fein et al.,2006). Commonly, multiple females enter and breed in the same seeds,sometimes even through the same entrance hole, allowing for both inbreeding andoutbreeding.
Behavioral studies also showed that these beetles will outbreedunder some conditions, suggesting that they may have a ‘mixed mating’ system.Two factors that are likely to affect breeding opportunities are climate(variable temperatures in the temperate latitudes for example) and seedproduction (either seasonally or episodically)(Holzman et al., 2009). EnvironmentThe circadian clock is an internalapparatus in most living organisms which is used to measure time for themanagement of physiological and behavioral processes that occur periodically.In many cases, the clock is based on the circadian rhythms, which operatearound a 24-hour cycle and match the activity of the organism to the changingeffect of the sun throughout the day. Like the sun, other external signals suchas temperature, biochemical materials or interactions between organisms orindividuals can also adjust the clock and adapt it to the changing environment.Depending on the life cycle of each different species, the circadian clock hasa genetic basis and it plays an important role in the organism’s development. Endosymbionts- intimate ‘living together’ of dissimilarorganisms (de Bary 1879in Werren & O’Neill,1997).
Symbioticmicroorganisms are extremely widespread in nature, having intimate and oftenobligatory associations with their `host’ species. Various facultativeendosymbionts are known as arthropods ‘reproduction manipulators’ (Zchori-Fein et al.,2006). Werren andO’Neill (1997) reviewed these manipulations and categorized theirinfluences: (1) increase host fitness, (2) increase production of infectedfemales, (3) decrease fitness of un-infected hosts, (4) decreasing sex ratio ofun-infected hosts and (5) increasing transmission rate of endosymbionts. Eukaryotes,bacteria or viruses as a parasites, may affect the mate choice processes andthe mechanisms of mating decisions can determine whether they can be consideredas agents of selection (Beltran-Bech& Richard, 2014). While PCR analysis of C.
dactyliperda eggsshowed that Wolbachia and Rikettsia are present, both mated andunmated females who were treated with antibiotics, stopped producing eggs (Zchori-Fein et al 2006).