Person:

Edgecombe, Gregory

A well-corroborated morphological scheme of interrelationships for centipedes, once broadly accepted, has been in conflict with molecular data with respect to deep branching events. Expanded taxonomic coverage compared to previous analyses adds longer fragments for 28S rRNA and a structural alignment as part of a sample of four genes (two nuclear ribosomal and two mitochondrial) for 111 extant species; these sequence data are combined with morphology under parsimony and maximum likelihood, exploring both traditional multiple sequence alignment and direct optimization approaches. Novel automated procedures to incorporate secondary structure information are also explored. The molecular data in combination yield trees that are highly congruent with morphology as regards the monophyly of all centipede orders as well as the major groups within each of the large orders. Regardless of the optimality criterion or alignment strategy, the Tasmanian/New Zealand Craterostigmomorpha is resolved in a different position by the molecular data than by morphology. Addition of morphology overturns the placement of Craterostigmomorpha in favour of the traditional morphological resolution and eliminates the need to posit major character reversals with respect to developmental mode and maternal care. Calibration of the tree with Palaeozoic and Mesozoic fossils for a relaxed clock analysis corroborates the palaeontological signal that divergences between centipede orders date to the Silurian and earliest Devonian, and familial divergences are likewise almost wholly Palaeozoic.

The only well-documented data on female brooding posture in the geophilomorph family Mecistocephalidae come from Dicellophilus carniolensis (C.L. Koch, 1847), in which the mother coils around the eggs and hatchlings with the dorsal surface outwards. This posture is shared by Craterostigmomorpha and Scolopendromorpha but not by other Geophilomorpha (united as Adesmata), which coil with the ventral surface outwards. The change in brooding behaviour has been thought to coincide with the evolution of ventral glandular pores in Adesmata and defends the basal split of Geophilomorpha into Placodesmata (Mecistocephalidae alone) and Adesmata. However, a brood of another mecistocephalid, Mecistocephalus togensis (Cook, 1896), documented in situ in Cameroon, shows the mother to guard the hatchlings with the ventral surface outwards, in the manner of Adesmata rather than that seen in Dicellophilus. This observation suggests that the brooding posture may be more subject to convergence or evolutionary reversal than previously expected.

The South Pacific is a biodiverse region of extreme evolutionary importance because it harbors ancient lineages and recent radiations. However, few population-level studies of genetic variation have been conducted in the land masses of this region. Likewise, the number of population-level studies using myriapods as models is extremely small. In this article, we compare the genetic structure of two species of centipedes in the genus Cryptops endemic to the South Pacific, one from a continental island, the other from oceanic islands. The level of genetic diversity and structure in C. pictus, a species endemic to New Caledonia, is much higher than in C. niuensis in Fiji and Vanuatu, despite the fact that C. niuensis is spread across two different archipelagos and several islands. The most likely explanation is the relatively young age of the remnants of the Vitiaz Arc (Fiji and Vanuatu) compared to New Caledonia. Using the emergence of Fiji-Vanuatu as a calibration point, C. pictus is estimated to have diverged by 23.4 Mya (upper 95% confidence interval) with a mean estimate of 11.7 Mya versus the 9.7 Mya of C. niuensis. Considering the absence of shared sequences between specimens from different sampling sites and the high genetic structuring within populations, C. pictus appears to be an ideal candidate to assess historical processes at a micro-evolutionary scale in New Caledonia.

Relative to its diversity (34 genera, 700 species), Scolopendromorpha has been undersampled in molecular phylogenetic analyses compared with the other chilopod orders. Previous analyses based on morphology have not resolved several key controversies in systematics and evolutionary morphology unambiguously. Here we apply new molecular and morphological data to scolopendromorph phylogenetics, with a focus on the evolution of blindness. The taxonomic sample includes 19 genera, many lacking previous molecular data, and diverse, cosmopolitan genera of Scolopendridae are sampled by multiple species. Phylogenetic analysis with Direct Optimization used 94 morphological characters and ca. 4.5 kb of sequence data from two nuclear (18S and 28S rRNA) and two mitochondrial (16S rRNA and COI) loci. A single most-parsimonious cladogram selected after sensitivity analyses resolves Scolopendromorpha as monophyletic, and divides it into a blind clade of three families (Plutoniumidae, Cryptopidae, Scolopocryptopidae) and its ocellate sister group, Scolopendridae. Some species-rich, cosmopolitan genera (Cormocephalus, Otostigmus, Scolopendra) in Scolopendridae are non-monophyletic, and in several instances (e.g. New and Old World Scolopendra) relationships are more congruent with geographical distributions than with traditional classifications. The tribe Asanadini is particularly subject to parameter-sensitivity, nesting in the combined analysis within Scolopendrini but as sister to all other Scolopendrinae for molecular data alone. The total-evidence tree unambiguously optimizes trunk segmentation: a 23-segmented trunk has a single origin in the blind clade.

The advent of numerical methods for analyzing phylogenetic relationships, along with the study of morphology and molecular data, have driven our understanding of animal relationships for the past three decades. Within the protostome branch of the animal tree of life, these data have sufficed to establish two major clades; Ecdysozoa, a clade of animals that all moult, and Spiralia (often called Lophotrochozoa), a clade whose most recent common ancestor had spiral cleavage. In this chapter, we outline the current knowledge of protostome relationships and discuss future perspectives and strategies to increase our understanding of relationships within the main spiralian clades. Novel approaches to coding morphological characters are a pressing concern, best dealt with by scoring real observations on species selected as terminals. Methodological issues, such as the treatment of inapplicable characters and the coding of absences, may require novel algorithmic developments. Taxon sampling is another pressing issue, as terminals within phyla should include enough species to represent their span of anatomical disparity. Furthermore, key fossil taxa that can contribute novel character state combinations, such as the so-called 'stem-group lophotrochozoans', should not be neglected. In the molecular forum, EST-based phylogenomics is playing an increasingly important role in elucidating animal relationships. Large-scale sequencing has recently exploded for Spiralia, and phylogenomic data are lacking from only a few phyla, including the three most recently discovered animal phyla (Cycliophora, Loricifera, and Micrognathozoa). While the relationships between many groups now find strong support, others require additional information to be positioned with confidence. Novel morphological observations and phylogenomic data will be critical to resolving these remaining questions. Recent EST-based analyses underpin a new taxonomic proposal, Kryptrochozoa (the least inclusive clade containing the Brachiopoda and Nemertea).

Twenty years after its proposal, the monophyly of molting protostomes—Ecdysozoa—is a well-corroborated hypothesis, but the interrelationships of its major subclades are more ambiguous than is commonly appreciated. Morphological and molecular support for arthropods, onychophorans and tardigrades as a clade (Panarthropoda) continues to be challenged by a grouping of tardigrades with Nematoida in some molecular analyses, although onychophorans are consistently recovered as the sister group of arthropods. The status of Cycloneuralia and Scalidophora, each proposed by morphologists in the 1990s and widely employed in textbooks, is in flux: Cycloneuralia is typically non-monophyletic in molecular analyses, and Scalidophora is either contradicted or incompletely tested because of limited genomic and transcriptomic data for Loricifera, Kinorhyncha and Priapulida. However, novel genomic data across Ecdysozoa should soon be available to tackle these difficult phylogenetic questions. The Cambrian fossil record indicates crown-group members of various ecdysozoan phyla as well as stem-group taxa that assist with reconstructing the most recent common ancestor of panarthropods and cycloneuralians.

Phylogenetic relationships in the lithobiomorph family Henicopidae are analysed based on sequence data for five molecular markers and 58 morphological characters. The character sample includes two nuclear ribosomal genes (complete 18S rRNA and the D3 region of 28S rRNA) and three mitochondrial genes, two ribosomal (16S rRNA and 12S rRNA) and one protein-coding (cytochrome c oxidase subunit I). Terminal taxa include six outgroup species of Lithobiidae and 41 exemplars of Henicopidae representing 33 species. Analysis of the combined dataset using Direct Optimization and exploring stability of clades to a range of gap:substitution and transversion:transition costs yields a most congruent (minimal ILD) cladogram that is largely congruent with the shortest molecular cladogram. Morphology, however, contributes additional nodes to the strict consensus of all explored parameters. The morphological cladogram resolves the Oriental Shikokuobius as sister to a gondwanan clade of Anopsobiinae, whereas the sequence data place Shikokuobius as sister to Henicopinae or all Henicopidae. Henicopinae sensu Attems is monophyletic for combined analyses for all parameters, being retrieved amongst the shortest morphological cladograms and by all molecular parameter sets, but the traditionally defined Zygethobiini is polyphyletic for several parameter sets. For the most congruent parameters, the nearctic Zygethobius is resolved in the expected position as sister to Henicopini, but the oriental Cermatobius nests within the Henicopini. Henicopini divides into two clades: one unites Henicops and Lamyctes (stable for all combined analyses as well as for morphological and molecular data alone) and the other unites the four gondwanan subgenera of Paralamyctes. In most analyses, South African species of Paralamyctes unite as a clade, with Cape endemics each others’ closest relatives. Most parameter sets for the molecular and combined data resolve a group that includes the Australasian Paralamyctes (Thingathinga) and P. (Haasiella) together with Patagonian species formerly placed in P. (Nothofagobius).

The interrelationships of the four classes of Myriapoda have been an unresolved question in arthropod phylogenetics and an example of conflict between morphology and molecules. Morphology and development provide compelling support for Diplopoda (millipedes) and Pauropoda being closest relatives, and moderate support for Symphyla being more closely related to the diplopod-pauropod group than any of them are to Chilopoda (centipedes). In contrast, several molecular datasets have contradicted the Diplopoda–Pauropoda grouping (named Dignatha), often recovering a Symphyla–Pauropoda group (named Edafopoda). Here we present the first transcriptomic data including a pauropod and both families of symphylans, allowing myriapod interrelationships to be inferred from phylogenomic data from representatives of all main lineages. Phylogenomic analyses consistently recovered Dignatha with strong support. Taxon removal experiments identified outgroup choice as a critical factor affecting myriapod interrelationships. Diversification of millipedes in the Ordovician and centipedes in the Silurian closely approximates fossil evidence whereas the deeper nodes of the myriapod tree date to various depths in the Cambrian-Early Ordovician, roughly coinciding with recent estimates of terrestrialisation in other arthropod lineages, including hexapods and arachnids.