Cytogenetic and anatomical study reveals chromosome count and reproductive traits of the mealybug Orbuspedum machinator

A cytogenetic and anatomical study of the legless mealybug Orbuspedum machinator found that females have a diploid chromosome number of 18, a complement not previously reported among the legless mealybugs known to the author. The species is associated with conical domiciles made from wax and densely packed fungal hyphae. Females inhabit these structures, which grow alongside the insects as the mealybugs supply the fungus with honeydew.

In November 2023, the author revisited the species’ type locality in southern Thailand and collected gravid females for cytogenetic and reproductive studies. Chromosome preparations from female embryos showed 18 chromosomes of broadly similar length. About half of the embryos studied contained cells with the characteristic Lecanoid heterochromatinization of the paternal chromosome set, a pattern associated with the unusual meiosis of lecanoid scale insects. In this system, paternal and maternal chromosome sets segregate during the second meiotic division, and only maternal-origin nuclei produce sperm.

The paternal heterochromatinization was interpreted as evidence of bisexual reproduction in the studied population, although adult males and male larvae were not found. The author proposed that the much smaller males may live separately from the larger females in different parts of a host plant or on different plants, as is known to occur among scale insects. Anatomical examination of the available females found a reproductive system resembling that of other studied legless mealybugs: paired lateral oviducts join an enlarged common oviduct, while a spermatheca and accessory glands connect to its proximal portion.

The study also documented complete ovoviviparity, with embryonic development occurring inside the ovarioles and oviducts. After hatching, primolarvae leave the maternal fungal domicile through its apical opening. These findings concern a collection of females and embryos from one population in Thailand, and the absence of observed males means that the proposed separate male lifestyle remains an explanation rather than a directly documented result. The reported symbiotic association with a sooty mold of the genus Capnodium was also described previously as unusual among scale insects and other animals known to the author.

Subject of Research: Karyotype and reproductive anatomy of the symbiotic mealybug Orbuspedum machinator

Article Title: Karyotype and reproductive traits of the unique symbiotic mealybug Orbuspedum machinator G.-Z. ( Homoptera , Coccinea )

Article References: Gavrilov-Zimin, I. A. (2023). Karyotype and reproductive traits of the unique symbiotic mealybug Orbuspedum machinator G.-Z. (Homoptera, Coccinea). Comparative Cytogenetics, 17, 283-286. https://doi.org/10.3897/compcytogen.17.116550

Image Credits: AI Generated

DOI: 10.3897/compcytogen.17.116550

Keywords: Orbuspedum machinator, mealybug, cytogenetics, karyotype, reproductive biology, ovoviviparity, Lecanoid heterochromatinization, fungal symbiosis, reproductive, traits, unique, symbiotic