differences that might be uncovered between populations or species must be of
profound importance because of the low within-population component of
variability (Avise, 2004).
Although Muller (1950) predicted that only one locus in 1,000 (0.1%) would
prove to be heterozygous in a typical individual (in Avise, 2004), proportions of polymorphic loci
in a population range generally from 10 to 20% in vertebrates and from 25 to
50% in invertebrates (Selander
& Kaufmen, 1973). The mean heterozygosity that found for 24 insect
species was 0.155, for another 24 invertebrates species was 0.1507 and for 22 Vertebrates
species 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 a
mutant gene in a diploid and a haplodiploid population led to the conclusion
that if the populations have equal numbers of genes, the values of fixation probability
are approximately equal, but if the populations have equal numbers of
individuals, the fixation probability in the haplodiploid population may reach
a maximum of 1.33 times that in the diploid population (Lester & Selander,
1979). A large
part of the genetic variability in natural populations may be due to balanced
polymorphism. For sex-linked loci in the haplodiploid species, there are five
genotypes to consider instead of the familiar three. Following LI (1967), the
genotypes for a pair of alleles are: AA Aa aa A a (Crozier, 1970).
systems in plants and animals vary, and may occur at any point on a continuum
between the two extremes of self-fertilization (an extreme case of inbreeding)
and outbreeding (Gottlieb et
al., 2009). The effect of inbreeding on fitness is a major theoretical
and empirical issue in evolutionary and conservation biology (Pray & Goodnight, 1995).
There are two classes of
effects, which are ascribed to inbreeding: 1) a decline in all elements of
vigor (e.g. weight, fertility, vitality) and 2) an increase in uniformity
within the inbred stock, correlated with an increase in prepotency in outside
1922). Inbreeding events in a normally outbred
population can lead to elevated levels of homozygosity, which may lead to
inbreeding depression (i.e., a reduction in fitness of inbred offspring
relative to progeny from unrelated parents) that is largely due to the
expression of deleterious recessive alleles (Henter, 2003). In some highly inbred animals it
has been shown that there is a preference to produce a precise sex ratio than
to produce a binomial sex ratio, although the appearance of a precise sex ratio
may 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 are
identical by descent, i.e., that an individual is homozygous for an ancestral
allele by inheritance and not by mutation (Rédei, 2008). The level of inbreeding depression in a
population is an essential parameter in models of plant and animal mating system
evolution. If increases (in the level of inbreeding), as measured by Wright’s
inbreeding coefficient F, exceed 1% per generation, then a population is at
risk for serious inbreeding depression. However, inbreeding depression may not
be a constant parameter that is identical for all populations of all species.
Indeed, even Darwin (1876) reported inbred lines that actually increased in
fitness relative to an outbred control (Pray & Goodnight, 1995).
various insects, the differentiation between the sexes is a matter of quantity
of genetic material: females are diploid, while males are haploid. Fertilized
eggs yielded a diploid daughter and males develop from
Therefore, haplodiploids may suffer less from inbreeding depression due to recessive
deleterious 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 highly
inbred, haplodiploidy parasitic wasps, mated females may produce most females
and exactly one male. In other cases, they may produce larger broods with the number of
males having lower than binomial variance. Therefore,
mated females are able to
control their population sex ratio and balance the product of the unmated
females that produce males only. Natural selection should favor the production
of the number of males that maximizes the mean number of inseminated females
emerging from each brood (Green
et al., 1982).
For polymorphism, although in
the haplodiploidy case values have to be correct for five fitnesses for polymorphism
(AA Aa aa A a), it seems that there is no
significant difference between the numbers of polymorphisms possible at five or
three genotypes (Crozier, 1970). In contrast, haplodiploidy reduces
recombination and dominance effects and causes temporal oscillatory changes in
gene frequencies between the genders in the populations of a specie. Moreover in
this system, linkage between genes and other loci should be increased because
there is no opportunity for crossing over or chromosome segregation in males (Lester & Selander,
1979). Increase those
linkages may lead to a decrease individual’s heterozygosity, and later on it may
affect the heterozygosity of whole population.
One way to assess
heterozygosity is to use DNA microsatellites
(as an example of genetic markers) that are powerful tools for studying mating
systems, populations’ structures and for measuring paternity also for the
diploid females in extreme inbreeders haplodiploidy
species. The high variability, neutrality, codominance, and Mendelian
inheritance of these markers make them well suited for investigating features
of population genetics. They can be used to reconstruct pedigrees, calculate
rates of inbreeding for natural populations, estimate parentage, and reliably
infer demographic gene flow (Rácz
et al., 2015; Berg
et al., 2003; Borquez & Brante, 2017).
bark beetle family Scolytinae, regular inbreeding by sib-mating is one of the
most successful ecological strategies. With currently 119 species assigned to
it, Coccotrypes are found breeding in an exceptional variety of non-standard
woody 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 Phoenix
and are counted as one of approximately 1,400 described species in the Xyleborini
tribe of predispersal, sib-mating beetles (Holzman et al., 2009).
The sex determination mechanism of this strongly
inbreeding species is haplodiploidy and can be further classified as
‘Arrhenotokous’: where the haploid dwarf males are produced from unfertilized
eggs and they may mate with their diploid mothers or sisters, prior to
dispersal. The offspring sex ratio of mated
females 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 male
offspring. This in turn might lead to a reduction in their offspring’s success
in encountering mating opportunities, thus incurring a reproductive cost (Gottlieb et al., 2014). Theoretical
and empirical studies indicated a selective advantage to mated females in
highly inbred haplodiploid populations where females can control precisely the gender
of their offspring. However, the occurrence of a high frequency of unmated
females in structured populations is not unusual for haplodiploids (Green et al., 1982;
Gottlieb et al., 2014).
C. dactyliperda reproduction
revolves around the date fruit’s stone. The adult female chews a round
bore-hole (1 mm diam.) in the fruit (Fig. 1), causing it to drop 1 or 2 days
later. The dwarf males are unable to penetrate the stones. Eggs are laid in
galleries 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 alone
in a seed (Gottlieb et
al., 2014). A newly emerged unmated female colonizes a new date
stone 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 and
outbreeding. Behavioral studies also showed that these beetles will outbreed
under 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 seed
production (either seasonally or episodically)
(Holzman et al., 2009).
The circadian clock is an internal
apparatus in most living organisms which is used to measure time for the
management of physiological and behavioral processes that occur periodically.
In many cases, the clock is based on the circadian rhythms, which operate
around a 24-hour cycle and match the activity of the organism to the changing
effect of the sun throughout the day. Like the sun, other external signals such
as temperature, biochemical materials or interactions between organisms or
individuals can also adjust the clock and adapt it to the changing environment.
Depending on the life cycle of each different species, the circadian clock has
a genetic basis and it plays an important role in the organism’s development.
Endosymbionts- intimate ‘living together’ of dissimilar
organisms (de Bary 1879
in Werren & O’Neill,
microorganisms are extremely widespread in nature, having intimate and often
obligatory associations with their `host’ species. Various facultative
endosymbionts are known as arthropods ‘reproduction manipulators’ (Zchori-Fein et al.,
2006). Werren and
O’Neill (1997) reviewed these manipulations and categorized their
influences: (1) increase host fitness, (2) increase production of infected
females, (3) decrease fitness of un-infected hosts, (4) decreasing sex ratio of
un-infected hosts and (5) increasing transmission rate of endosymbionts. Eukaryotes,
bacteria or viruses as a parasites, may affect the mate choice processes and
the mechanisms of mating decisions can determine whether they can be considered
as agents of selection (Beltran-Bech
& Richard, 2014). While PCR analysis of C. dactyliperda eggs
showed that Wolbachia and Rikettsia are present, both mated and
unmated females who were treated with antibiotics, stopped producing eggs (Zchori-Fein et al 2006).