Abstract
Metamorphosis is a key innovation allowing the same species to inhabit different environments and accomplish different functions, leading to evolutionary success in many animal groups. Astigmata is a megadiverse lineage of mites that expanded into a great number of habitats via associations with invertebrate and vertebrate hosts (human associates include stored food mites, house dust mites, and scabies). The evolutionary success of Astigmata is linked to phoresy-related metamorphosis, namely the origin of the heteromorphic deutonymph, which is highly specialized for phoresy (dispersal on hosts). The origin of this instar is enigmatic since it is morphologically divergent and no intermediate forms are known. Here we describe the heteromorphic deutonymph of Levantoglyphus sidorchukae n. gen. and sp. (Levantoglyphidae fam. n.) from early Cretaceous amber of Lebanon (129 Ma), which displays a transitional morphology. It is similar to extant phoretic deutonymphs in its modifications for phoresy but has the masticatory system and other parts of the gnathosoma well-developed. These aspects point to a gradual evolution of the astigmatid heteromorphic morphology and metamorphosis. The presence of well-developed presumably host-seeking sensory elements on the gnathosoma suggests that the deutonymph was not feeding either during phoretic or pre- or postphoretic periods.
Highlights
The evolution of metamorphosis is thought to have generated an incredible diversity of organisms, allowing them to exploit different habitats and perform different functions at different life stages[1,2,3,4,5]
Oral feeding usually does not occur during phoresy in acariform mites, obligate non-oral "retro-feeding" has been reported for a few astigmatid heteromorphic deutonymphs associated with insects and vertebrates[30,31]
Extant taxa lack any intermediates that would give a clue about their early morphological evolution, so it may appear that the unique morphology of the heteromorphic deutonymph originated via a single punctuated change, leading to dramatic evolutionary modifications associated with phoresy and the loss of oral feeding
Summary
The evolution of metamorphosis is thought to have generated an incredible diversity of organisms, allowing them to exploit different habitats and perform different functions at different life stages[1,2,3,4,5]. We use paleontological evidence to evaluate whether gradual or punctuated evolution was the main process that produced the complex life-cycle in astigmatid mites These mites have two disparate phenotypes accomplishing different functions: (1) dispersing heteromorph (deutonymph that lost oral feeding and disperses via phoresy on hosts) and (2) feeding/reproductive homeomorphs (larva, protonymph, tritonymph, adults). Oral feeding usually does not occur during phoresy in acariform mites, obligate non-oral "retro-feeding" (i.e., via the anus or genital papillae) has been reported for a few astigmatid heteromorphic deutonymphs associated with insects and vertebrates[30,31]. These deutonymphs, are both phoretic and parasitic. As R. Norton[28] has put it in relation to a broader question on the origin of the astigmatid mite life history: "If gradual physiological
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