ORIGINAL ARTICLE Partial characterization of the X chromosome of Diuraphis noxia (Hemiptera: Aphididae) Louis STEYN, Anandi BIERMAN, N. Francois V. BURGER and Anna-Maria BOTHA Genetics Department, Faculty of AgriSciences, Stellenbosch University, Matieland, South Africa Abstract We present the first sequencing results after separation of the X chromosome of Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), the largest known X chromosome described to date, using flow cyto- metry. The X chromosome of D. noxia is 0.1824 pg (1C) and an estimated 178.4 Mb (1C) in size. Map- ping confirmed that the X chromosome contains 13,799 protein coding genes, but with a slight bias towards GC richness when compared to the complete D. noxia genome. Key words: Aphididae, Diuraphis noxia, flow sorting, Hemiptera, Illumina HiSeq 2000 system, Russian wheat aphid, sex chromosome. INTRODUCTION Arthropods are known as the most speciose clade of animals on earth. The Insecta class, which is the major- ity of arthropods, is hypothesized to hold 75% of all animals (Gaston 1991). Arthropod genome sizes show considerable diversity, with the largest reported to date being that of the grasshopper (Orthoptera: Neoconocephalus triops L.) (1C = 7.125 (M)/7.752 (F) Mb; 7.93 pg) and the smallest being the two- spotted spider mite (Trombidiformes: Tetranychus urticae) (1C = 90.7 Mb; 0.09 pg) (Johnston et al. 2007; Hanrahan & Johnston 2011). The karyotypic structure of the aphid genome has been found to vary subs- tantially among the species, which makes referring to phylogeny irrelevant. In some aphid genera the chromo- some number seems to be stable, but in others it differs greatly, mainly because of successive autosomal dissoci- ations with slight or no accompanying morphological or biological change (Manicardi et al. 2015). The chromosomes of Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), like that of other aphids and some hemipteroid insects, are holocentric. Holocentric chromosomes lack localized centromeres, with centromeric activity being diffused along the length of the chromosome. As a result, during the later stages of prophase and metaphase every condensed chromo- some appears rod-like, or, if treated with colchicine, as a pair of chromatids aligned in parallel with a uniform gap between them (Blackman 1987). Holocentric chro- mosomes present a big disadvantage for cytogenetic studies and karyotype comparisons in that they do not possess any primary and/or secondary constrictions. This means that conventionally stained preparations can only be identified on the basis of size. Thus, the only fac- tors that can be used to detect differences in karyotype structure are the number of chromosomes or measurable structural heterozygosity (Manicardi et al. 2015). Novotná et al. (2011) established that the karyotype of female D. noxia consists of 10 chromosomes (2n = 10) with four pairs of autosome chromosomes and a pair of large sex (X) chromosomes. They also noted that the male D. noxia only possess one sex (X) chromosome and therefore has a karyotype of 2n = 9. For that reason the D. noxia sex chromosome system has been identified as XX/X0, as in a number of aphid species (Jaquiery et al. 2012). This system never has a Y chromosome, which is essential for parthenogenesis or asexual reproduction (Kuznetsova & Shaposhnikov 1973). Aphids are known for their large X chromosomes, as reported by Novotná et al. (2011) for D. noxia. The reason for the existence of large sexual chromosomes relative to the autosomes in aphids and their origin are still unknown (Mandrioli et al. 1999; Bizzaro et al. 2000). Correspondence: Anna-Maria Botha, Genetics Department, Faculty of AgriSciences, Stellenbosch University, Private Bag X1, Matieland 7601, South Africa. Email: ambo@sun.ac.za *Declaration of conflict of interest The authors declare that they have no competing interests. Received 3 September 2018; accepted 20 October 2019; first published 10 February 2020. © 2020 The Entomological Society of Japan Entomological Science (2020) 23, 86–94 doi: 10.1111/ens.12401