|Gynandromorph of Papilio glaucus (M=L/F=R), photographed by James Adams|
|Cluster of Icerya purchasi photographed by Colette Micallef (host plant unspecified)|
|You thought that the life cycle of Micromalthus debilis was complicated? Get a load of this|
In I. purchasi, males are rare but not unknown; the bulk of the population (>90%) consists of self-fertilizing "hermaphrodites" in functional terms, but females in fact (Hughes-Schrader, 1925), with spermatozoa produced by haploid tissue (Royer, 1975). In the case of I. purchasi, at least, this haploid tissue is established from excess parental sperm, which, rather than fertilize oocytes, establish themselves transovarially in female oocysts and proliferate within diploid female tissues. As Normark (2003) put it, "hermaphroditism may be an inadequate term with which to describe this situation." Rather, female cottony cushion scales are infected with a vertically-transmitted clonal lineage of males reduced to a germ line. I. purchasi females breed with this "permanent cancer of the[ir] germ line" (Normark, 2003), which due to the lack of recombination inherent in haploid males' conception is (in genetic terms) their own father incarnate as a teratoma.
|Schematic of reproductive possibilities in hermaphroditic Icerya spp. (Gardner & Ross, 2011)|
Icerya does not contain the only examples of dizygotic (Gavrilov & Kuznetsova, 2007) tissue in coccoids, although the other two examples have an entirely separate adaptive justification: namely, to vertically transmit bacteriomes, a descriptive term referring to specialized tissues (consisting of bacteriocytes) that withhold endosymbionts. In Sternorrhyncha, it is generally assumed that these bacteria provide essential amino acids absent from a diet (plant sap) that consists almost exclusively of polysaccharides (Thao et al., 2002). In the monobasic (Hodgson and Hardy, 2013) Putoidae, which unlike Icerya are diplodiploid (Hughes-Schrader, 1944), maternal bacteriocytes invade the nascent embryo and propagate therein, bacteria and all (Buchner, 1965), thus giving rise to a bacteriome of maternal tissue. Cytogenesis that has dizygotic products is termed Schrader fusion (Gavrilov & Kuznetsova, 2007).
|β-(blue) and γ-(red) proteobacteria, both without (L) and within (R) Planococcus citri oocyte|
|Diagram of diaspidid life cycle; note triploidy of polar body that undergoes Schrader fusion (Normark, 2004)|
Why the multiple dizygotic systems cited above? And why among the Coccoidea alone does this occur, among all the diversity that is the Metazoa? To be brief, we do not know. Normark (2004) hypothesizes that polar body-based Schrader fusions act to conceal the sex of offspring from endosymbionts, which due to strictly vertical transmission are incentivized to skew sex ratios towards females. In this scenario, Schrader fusion results from genetic conflict between mutualistic symbionts. However, experimental work is required for any substance to attach to this intriguing idea.
*A term for the clade including the Hexapoda, and the "Crustacea" from within which the hexapods descend.
†A category into which a confirmed bacterial taxon can be placed pending formal description.
‡Nuclei in a proliferating embryo that have not undergone cleavage, such that they are surrounded by distinct halos of cytoplasm but have no cell walls.
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