Origin of the novel chemoreceptor Aesthetasc “Y” in Ostracoda: morphogenetical thresholds and evolutionary innovation

Abstract

The morphology and developmental processes of the two types of ostracod chemoreceptors, the Aesthetasc "Y" and the "Grouped setae," were compared. Cypridoidea and Pontocypridoidea, belonging to Cypridocopina, have a large baseball bat-like seta as an autapomorphic character on the second antenna, whereas most ostracod taxa with plesiomorphic characters bear "Grouped setae" consisting of multiple setae on the second antenna. Their budding positions, morphology, and ontogenetic changes were compared, and our deduction is that the Aesthetasc "Y" originated from "Grouped setae-like" organ in the Paleozoic. The morphogenetic processes in the molting period of these chemoreceptors were compared at the cellular level. The observations suggest that the "Grouped setae" are formed by hypodermal cells and share sheath cells corresponding to those of the Aesthetasc "Y" as a common constraint in the molting process of setae. We conclude that modification of the morphogenetic processes in the molting period of the "Grouped setae" gave rise to the Aesthetasc "Y" as a novel organ in the evolutionary pathway of the Ostracoda.

Exceptionally preserved ostracodes from a Middle Miocene palaeolake, California, USA

“…Remarks: The 'baseball bat' morphology of the aesthetasc on the second antenna supports the assignment of Raepula to the Cypridoidea, its morphology being an apomorphy for Cypridocopina (Kaji and Tsukagoshi, 2008). Raepula has a smooth carapace; it lacks a distal triangle on the furca; the posterior seta of the furca is relatively close to the base of the terminal claw; and the basal segments of the 6th appendage bears two setae.…”

Macrocyprididae (Ostracoda) from the Southern Ocean: taxonomic revision, macroecological patterns, and biogeographical implications

“…The absence of pores in these setae shows that they are most probably not chemosensors, but just mechanoreceptors. Consequently these setae should not be regarded as aesthetascs (as used by Danielopol, 1978 and Maddocks, 1990), but as ‘grouped setae’ (as used by Rossetti & Martens, 1998; Smith & Martens, 2000; Kaji & Tsukagoshi, 2008).…”

“…55M, N) and extremely modified in males. Similarly to aesthetasc Y (Andersson, 1975; Kaji & Tsukagoshi, 2008), these male setae are composed of three parts: (1) a long proximal (∼66% of total setal length), with smooth surface (Fig. 55D); (2) a short intermediate part (∼12% of total setal length) that is very finely striate laterally and presents many subspherical prominences medially (Fig.…”

Obituary: Tomonari Kaji (1982–2019), evolutionary morphologist extraordinaire

“…Many of Tomonari's publications, on an impressive array of arthropod forms, show the great value of this two‐pronged approach for understanding the developmental or evolutionary origins of some fascinating morphological innovations, for example: A highly unusual sensory structure derived from the fusion of multiple setae in ostracods (Kaji & Tsukagoshi, ), the spectacular developmental transformation of a jointed mouthpart into a fully‐formed suction disc in branchiuran fish lice (Figure ; Kaji, Møller, & Tsukagoshi, ), transformation of body plates to yield an equally striking adhesive disk for host‐attachment in ectoparasitic copepods (Kaji et al, ), an utterly remarkable squirting head where mandibular muscles took over control of squirting as the mandibles were lost entirely in nasutitermine termite soldiers (Figure ; Kaji, Keiler, Bourguignon, & Miura, ), transformation of mandible form from simple particle‐processing to piercing carnivory in Viper copepods (Kaji et al, ), and his magisterial survey of joint morphology and muscle re‐alignment in caridean shrimp claws (Figure ; Kaji, Anker, Wirkner, & Palmer, ). The latter revealed a wholly novel energy storage mechanism for rapid claw closure that set the stage for the evolution of the spectacularly fast closing claws in snapping shrimp.…”

“…During his early postdoctoral years, Tomonari also made forays into philosophy (Kaji, 2012a(Kaji, , 2012bKaji, 2013). For example, he explained to me how he believed that the emergence of form during development was conceptually equivalent to the emergence of an idea during human Many of Tomonari's publications, on an impressive array of arthropod forms, show the great value of this two-pronged approach for understanding the developmental or evolutionary origins of some fascinating morphological innovations, for example: A highly unusual sensory structure derived from the fusion of multiple setae in ostracods (Kaji & Tsukagoshi, 2008), the spectacular developmental transformation of a jointed mouthpart into a fully-formed suction disc in branchiuran fish lice (Figure 2; Kaji, Møller, & Tsukagoshi, 2011), transformation of body plates to yield an equally striking adhesive disk for host-attachment in ectoparasitic copepods (Kaji et al, 2012), an utterly remarkable squirting head where mandibular muscles took over control of squirting as the mandibles were lost entirely in nasutitermine termite soldiers (Figure 3; Kaji, Keiler, Bourguignon, & Miura, 2016), transformation of mandible form from simple particle-processing to piercing carnivory in Viper copepods (Kaji et al, 2019), and his magisterial survey of joint morphology and muscle re-alignment in caridean shrimp claws (Figure 4; Kaji, Anker, Wirkner, & Palmer, 2018). The latter revealed a wholly novel energy F I G U R E 2 Schematic illustration of ontogeny of the first maxilla of a branchiuran crustacean (fish louse) as it transforms from a normal leg-like appendage with a hook in the early larvae (a) through to a fully functional suction disc with a distal podomere vestige in the attached juvenile (c; fig.…”