The number and nature of the metameric elements constituting the head is one of the highly controversial issues in developmental biology, morphology, and phylogenetics of arthropods [1,2]. The most controversial head structure is the arthropod labrum, or upper lip. Based on a great variety of approaches and evidence the labrum is interpreted as being a derived pair of limbs with unclear segmental affiliations, a simple outgrowth, an anterior segment or the anterior most non-segmental body terminus, the acron [1,3]. Another structure of unresolved nature is the paired lower lip which is called paragnaths in crustaceans or superlinguae/hypopharynx in hexapods and some myriapods [4,5]. Paragnaths have been described for malacostracans, branchiopods, copepods, ostracods, mystacocarids, and cephalocarids (e.g. Ref. [4,6-10]). They are located in the posterior area of the mandibles and form often the lower border of the pre-oral cavity [4,11,12] (Fig. 1). Paragnaths play a role in feeding and in some taxa they are movable and have a complex and appendage-like appearance [13]. Moreover, in many crustaceans they are embryologically formed as limb-bud like outgrowths (e.g. Ref. [14-18]).
The superlinguae of many hexapods and Pauropoda, Symphyla and Diplopoda among the myriapods form, together with the unpaired lingua, the hypopharynx, a tongue-like structure at the posterior of the pre-oral cavity [5,19-22]. As is the case for the crustacean paragnaths, the hypopharynx/superlinguae are involved in food processing and embryologically they are formed as two processes at the posterior stomodeal region [23-25].
Although these paragnathal/superlingual/hypopharyngeal structures are not so much a focus of the general head debate as is the labrum, they present a similar set of problems. First, it is not clear whether paragnaths and superlinguae are homologous (see Ref. [5,26,27]). Second, based on early development, the sometimes complex adult structure, and the innervation pattern some authors interpret the paragnaths as being derived from limbs (e.g. Ref. [26,28,29]) or as parts of limbs [12,30,31] whereas others dispute this [10,32]. Third, the segmental relation is seen controversially either as postoral lip not related to any particular segment [7], as part of the mandibular segment [10,33], the segment of the first maxillae [8,12], or even the second maxillae [9]. Hansen [28], Denis [34], Chaudonneret[26], Laverack [35], and Casanova [29] suggest that the paragnaths indicate an additional segment either between mandibular and maxillary segments [28] or the tritocerebral and mandibular segments [26,29,34,35], concluding that the arthropod or mandibulate head comprises one more segment than generally thought. Lauterbach [32] hypothesized the origin of the paragnaths in sternal folds ("sternale Falten") of ancestral arthropods. According to Lauterbach the paragnaths are the result of progressive bulging and fusion of sternal elements of the first post-oral head segments in the Mandibulata, though only in some crustacean taxa do these folded sternal formations ("Faltenbildungen") have an appendage-like appearance.
Here we address the problem of the nature and origin of crustacean paragnaths with a cell lineage approach. To resolve the segmental affinities of the paragnaths we study the cell lineage in the area around the mandibular segment of the freshwater beach-hopper, the amphipod crustacean Orchestia cavimana. This animal is well suited for this kind of investigation because it forms a pair of large buds of paragnaths during embryonic development [17]. Furthermore, as an amphipod representative its stereotypic cell division pattern in the post-naupliar region (segments of the first maxillae to the terminal segment of the pleon) is known (for this see Fig. 2D) and has been described up to the formation of morphological structures such as limbs, segments, and ganglia (Fig. 2E) [36]. In contrast to the well studied post-naupliar region, the processes of cell division and morphogenesis of the segments of the first and second antennae and the mandible (i.e. the naupliar region) are less known. This is due to the more irregular mode of cell division which, apart from some indications in the posterior mandibular region, does not show an obvious stereotypic pattern [36,37].
We combine the methods of 4D-microscopy
[
38] and the in-vivo labeling of single cells with the fluorescent dye DiI
[
39] to resolve the cell division pattern in the posterior naupliar region to trace the origin and formation of the paragnaths and other mouthparts by analyzing the clonal composition of the mandibular segment and adjacent areas.
It can be shown that the posterior region of the mandibular segment shows an unexpected degree of cell division determination with a reproducible cell lineage. The clear-cut results of our study shed new light on the segmentation pattern of crustacean heads by dismissing some older hypotheses on the origin and nature of paragnaths. The comparison and discussion of putative homologous structures in other arthropod taxa offer new perspectives on arthropod heads in general.
Answers to all your Biology Questions
