Assessing the phylogenetic utility of sequence heterochrony: evolution of avian ossification sequences as a case study
Introduction
The evolution of organismal morphology is a direct result of changes in organismal development (Haeckel, 1902; Gould, 1977; Hall, 1999). Heritable changes are directed by genetic changes while non-heritable changes are the result of alterations of epigenetic influences, such as climate.
Heterochrony has been an important concept for the examination of large-scale evolutionary and developmental changes, both as a system of measurement and description, and as a framework for permitting the discussion of evolutionary processes acting through development. Heterochrony encompasses the entire suite of possible changes in developmental timing that result in evolutionary change. This relatively straightforward evolutionary mechanism has been found to act across invertebrate and vertebrate animals and through all stages of development (Gould, 1977; Alberch et al., 1979; McKinney, 1988), and is considered to be an important pattern underlying morphological change. More recent work on heterochrony has divided it into two main components based on the type of developmental change (Smith, 2001). Growth heterochrony describes the differential growth rates and differential timing of onset and offset of somatic growth, generally compared to the onset of sexual maturity. The comparisons are usually bivariate. Growth heterochronies result in changes in size and shape of morphological structures. Sequence heterochrony describes evolutionary changes in the temporal order of developmental events (Smith, 2001). Sequence heterochronies are currently recognised as changes in a rank-ordered sequence of developmental events, but the utility of defining sequence heterochronies based upon absolute or stage-based temporal information has also been discussed (Smith, 2001).
Ossification sequences are species-specific patterns of bone formation that appear to exhibit some degree of conservation within species (Sheil, 2003). The sequences are not invariant, however, and differences occur within species (intraspecific polymorphism) (Sheil and Greenbaum, 2005), between closely related species (Prochel et al., 2008), and between higher taxonomic groups (Sánchez-Villagra, 2002). Ossification sequences of primary centres of ossification have not been found to exhibit strong phylogenetic signal when tested in an explicit context using objective methodologies (Sánchez-Villagra, 2002; Schoch, 2006; Sánchez-Villagra et al., 2008; Weisbecker et al., 2008). However, the goal of these studies was not to characterise ossification sequence changes between closely related taxa, but to highlight large sequence changes over distantly related taxa. Furthermore, the number of elements in each of the previous studies was relatively low (16–25 elements examined) and restricted in most cases to either the skull or the postcranium.
Extant birds provide a good system to study the evolution of ossification sequences. The majority of primary ossification centres in birds arises in ovo so differences in hatchling functions and behaviours are not expected to directly affect the sequence of appearance of primary ossification centres. Much of avian morphology is limited by the constraints imposed by flight, and most birds share a basic body plan making homology determinations straightforward. Although birds have been described as being extremely conservative in gross embryonic morphology (Kerr, 1919), and also in the timing of onset of ossification (Starck, 1993), heterochronic changes in external embryonic morphology and in ossification sequence have been observed (Price, 1938; Maxwell, 2008a, Maxwell, 2008b; Maxwell and Harrison, 2008). While the relationships within orders of extant birds may not be well resolved (Livezey, 1997; Donne-Goussé et al., 2002; Crowe et al., 2006; Kaiser et al., 2007), there is a consensus on the topology of the most basal divergences (Gibb et al., 2007; Livezey and Zusi, 2007; Slack et al., 2007). This creates a simple benchmark to test the accuracy of any topology derived using developmental data.
In this study, we test whether ossification sequence data contains phylogenetic signal in birds at the ordinal level, and we also reconstruct ossification sequence evolution in birds on an accepted topology using two methodologies that differ in computational speed and philosophical assumptions (see Methods). Finally, we discuss general, conserved features of avian ossification sequences.
Section snippets
Data acquisition
The order of appearance of the first ossification centre for 99 skeletal elements, both cranial and post-cranial (see TableS1 of the supplementary online Appendix at doi: 10.1016/j.zool.2009.06.002), was coded for 14 avian species: Anas platyrhynchos, Cairina moschata, Coturnix coturnix (two samples), Dromaius novaehollandiae (two samples), Gallus gallus (two samples), Larus argentatus, L. canus, L. ridibundus, Meleagris gallopavo, Phalacrocorax auritus, Somateria mollissima, Stercorarius skua,
Topology reconstruction
A phylogenetic analysis using event-pairs as characters analysed with parsimony yielded six most parsimonious trees with a length of 2161 event-pair changes, and 185 sequence heterochronies (consensus – see TableS2 of the supplementary online Appendix at doi:10.1016/j.zool.2009.06.002). 829 of the 4851 event-pairs were parsimony-informative. The resulting trees were identical, except for the variable placement of S. mollissima and the sample of C. coturnix described by Nakane and Tsudzuki (1999)
Ossification sequence and phylogeny reconstruction
Stercorarius skua is pulled basal to all other neognaths in the topology constructed using event-pairs (Fig. 1) due in part to late ossification of the ceratobranchials, a feature shared with palaeognaths. S. skua also exhibited delayed ossification of pedal phalanges IV:2 and 4 as reported for ratites, and an early ossification of manual phalanx IV:1 as in Struthio camelus. The ossification sequence of S. skua was taken from the literature (Maillard, 1948) rather than being based on personal
Conclusion
Ossification sequences are conserved and non-random in birds, both within and between species. However, sequence data from a broad taxonomic sample of birds were found to contain a weak phylogenetic signal, similar to the results of previous studies differing in both taxonomic scope and elements examined (Sánchez-Villagra, 2002; Schoch, 2006). The sheer volume of elements analysed in this study may have had the effect of diluting changes occurring over appropriate time scales with more rapidly
Acknowledgements
We would like to thank the Larsson–Carroll labs, as well as E. Abouheif and R. Carroll (supervisory committee of EEM) for discussion. NSERC and a Tomlinson Fellowship provided doctoral funding to EEM. FQRNT and NSERC provided masters and doctoral funding, respectively, to LBH. Laboratory support was provided from NSERC and a Canada Research Chair to HCEL. The comments of V. Weisbecker and two anonymous reviewers significantly improved this manuscript.
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