Editing Scale insect (section)

=== Reproduction and the genetics of sex determination ===

Scale insects show a very wide range of variations in the genetics of sex determination and the modes of reproduction. Besides [[sexual reproduction]], a number of different forms of reproductive systems are employed, including [[asexual reproduction]] by [[parthenogenesis]]. In some species, sexual and asexual populations are found in different locations, and in general, species with a wide geographic range and a diversity of plant hosts are more likely to be asexual. Large population size is hypothesized to protect an asexual population from becoming extinct, but nevertheless, parthenogenesis is uncommon among scale insects, with the most widespread generalist feeders reproducing sexually, the majority of these being pest species.<ref>{{cite journal |last1=Ross |first1=Laura |last2=Hardy |first2=Nate B. |last3=Okusu |first3=Akiko |last4=Normark |first4=Benjamin B. |year=2013 |title=Large population size predicts the distribution of sexuality in scale insects |journal=Evolution |volume=67 |issue=1 |pages=196–206 |doi=10.1111/j.1558-5646.2012.01784.x |pmid=23289572|doi-access=free }}</ref>

[[File:Drosicha_Lefroy.jpg|thumb|A winged male ''Drosicha'' sp.]]

Many species have the XX-XO system where the female is [[Ploidy|diploid]] and homogametic while the male is [[Heterogametic sex|heterogametic]] and missing a sex chromosome. In some [[Diaspididae]] and [[Pseudococcidae]], both sexes are produced from fertilized eggs but during development males eliminate the paternal genome and this system called paternal genome elimination (PGE) is found in nearly 14 scale insect families. This elimination is achieved with several variations. The commonest (known as the lecanoid system) involved deactivation of the paternal genome and elimination at the time of sperm production in males, this is seen in Pseudococcidae, [[Kerriidae]] and some [[Eriococcidae]]. In the other variant or ''[[Comstockiella]]'' system, the somatic cells have the paternal genome untouched. A third variant found in Diaspididae involves the paternal genome being completely removed at an early stage making males haploid both in somatic and germ cells even though they are formed from diploids, i.e., from fertilized eggs. In addition to this there is also true haplodiploidy with females born from fertilized eggs and males from unfertilized eggs. This is seen in the genus ''[[Icerya]]''. In ''[[Parthenolecanium]]'', males are born from unfertilized eggs but diploidy is briefly restored by fusion of haploid cleave nuclei and then one sex chromosome is lost through heterochromatinization. Females can reproduce parthenogenetically with six different variants based on whether males are entirely absent or not (obligate v. facultative parthenogenesis); the sex of fertilized v. unfertilized eggs; and based on how diploidy is restored in unfertilized eggs. The evolution of these systems are thought to be the result of intra-[[Intragenomic conflict|genomic conflict]] as well as possibly inter-genomic conflict with endosymbionts under varied selection pressures. The diversity of systems has made scale insects ideal models for research.<ref>{{cite journal |last1=Ross |first1=Laura |last2=Pen |first2=Ido |last3=Shuker |first3=David M. |date=2010 |title=Genomic Conflict in Scale Insects: the causes and consequences of bizarre genetic systems |journal=Biological Reviews |volume=85 |issue=4|pages=807–828 |doi=10.1111/j.1469-185X.2010.00127.x|pmid=20233171 |s2cid=13719072 }}</ref>