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My PhD thesis in the blog post format

I wrote the introduction of my PhD thesis as a series of blog posts.

I miss blogging. I began in 2006 and did it quite actively until 2011—when my posting frequency declined. During my PhD I was still drafting several posts, but ended up only publishing a few. I missed the sharing of ideas and the open discussion that a blog post can give. And this is why I decided to write the introduction of my PhD thesis as a series of blog posts.

Now that I have defended my PhD, I’ll post some sections of the text here. The thesis is accessible at the Bergen Open Research Archive (BORA – UiB):

Bruno C. Vellutini PhD Thesis

Vellutini, B.C., 2016. Comparative development of spiralian larvae. The University of Bergen. Available at:

The first section is online. I’ll post the remaining in the coming weeks. Enjoy, comment, disagree, re-use, etc :)

  1. Endless larval forms most beautiful: what a larva is (also at The Winnower)
  2. Larvae as the epitome of evolution (also at The Winnower)
  3. Spiral cleavage, an oblique matter (also at The Winnower)
  4. Segmentation, a question of boundaries (also at The Winnower)

4 replies on “My PhD thesis in the blog post format”

This text is a section of my PhD thesis.

The Latin word lārva means evil spirit, ghost or mask1. In the 18th century, the naturalist Carolus Linnaeus was the first to employ the word larva to describe a stage in the life of an animal in which its adult form is still hidden or masked (Linnaeus, 1767, p. 534). An exemplar case of this new biological meaning is the maggot—the larval stage of a fly—whose wormy form and life style truly differs from its flying adult stage.Not all larvae, however, are masked forms. The larval body of some marine snails2, for example, is very similar to its adult body, except for the dazzling presence of a ciliated velum, used by the larva to swim and gather food (Collier, 1997). In more general terms, larval stages are considered to be a modification of embryonic development usually characterized by a morphology and habitat that are disparate from the adult stage (Hall and Wake, 1999). Because embryonic development can change in a multitude of ways, as evidenced by the great diversity of larval forms in nature (see below), there is no precise definition of larva (Hickman, 1999; Strathmann, 1993). Thus in practice, what a larva is, is defined case by case according to the organism and to one’s research background.The majority of animals on this planet have a complex life cycle with one or more larval stages. Collectively, marine invertebrates represent a great part of the observed larval diversity. Molluscs have the veliger, a shelled larva with the ciliated velum mentioned above; echinoderms have the pluteus, a spaceship-like larva with eight food-capturing arms, and the brachiolaria, a free-swimming larva driven by body-length dancing arms; bryozoans have the cyphonautes, a paper-thin triangular larva that sails over kelp blades; crustaceans have the zoea, an armored larva that swims as if using a jet pack; nemerteans have the pilidium, a larva with lobes and lappets in the form of a deerstalker cap… and this list goes on. The diversity of larval forms is astonishing.Sample of the diversity of metazoan larval forms. Larvae are not to scale. Photos from the Cifonauta marine biology image database (Migotto and Vellutini, 2011).Most of these charismatic larval figures were discovered in the 19th century by the naturalist founders of comparative embryology (Hall and Wake, 1999). At the time, the ideas of Karl Ernst von Baer and Ernst Haeckel had great influence on the understanding of embryonic development (Guralnick, 2002; Hall, 2000). Ontogeny was seen as the unfolding of an immutable process that represents the evolutionary history of an organism—an idea known as recapitulation or Haeckel’s biogenetic law: “ontogeny is a rapid and shortened recapitulation of phylogeny.” (Gould, 1977; Haeckel, 1866).These influential ideas were directly challenged by the mere existence of larvae. Or more generally, challenged by the existence of differentiated developmental stages that are, at the same time, functionally adapted to their environment and morphologically diverse. Such impressive variety of larval forms instigated questions about the relationship between the embryonic development of an individual (ontogeny) and the evolutionary history of a lineage (phylogeny).Do larvae represent ancestral adult forms? How many times have larvae evolved? Are larval structures homologous or independently evolved? Soon, there was an urge to rationalize the diversity of larval forms into an evolutionary context.

Read the next section: Larvae as the epitome of evolution.

ReferencesCollier, J.R., 1997. Gastropods, the Snails. In S. F. Gilbert & A. M. Raunio, eds. Embryology: constructing the organism. Sinauer Associates, Inc., pp. 189–217.Gould, S.J., 1977. Ontogeny and phylogeny, Harvard University Press.Guralnick, R., 2002. A Recapitulation of the Rise and Fall of the Cell Lineage Research Program: The Evolutionary-Developmental Relationship of Cleavage to Homology, Body Plans and Life History. Journal of the history of biology, 35(3), pp.537–567. Available at:, E., 1866. Generelle Morphologie der Organismen, Georg Reimer, Berlin. Available at:, B.K., 2000. Balfour, Garstang and de Beer: The First Century of Evolutionary Embryology. American zoologist, 40(5), pp.718–728. Available at:;2.Hall, B.K. & Wake, M.H., 1999. Chapter 1 – Introduction: Larval Development, Evolution, and Ecology. In B. K. H. H. Wake, ed. The Origin and Evolution of Larval Forms. San Diego: Academic Press, pp. 1–19. Available at:, C.S., 1999. Chapter 2 – Larvae in Invertebrate Development and Evolution. In B. K. H. H. Wake, ed. The Origin and Evolution of Larval Forms. San Diego: Academic Press, pp. 21–59. Available at:, C., 1767. Systema Naturæ, Impensis direct. Laurentii Salvii. Available at:, A.E. & Vellutini, B.C., 2011. Cifonauta – marine biology image database. Cifonauta, an image database for marine biology. Available at: [Accessed December 16, 2015].Sars, M., 1837. Beitrag zur Entwicklungsgeschichte der Mollusken und Zoophyten. Archiv für Naturgeschichte, 3, pp.402–407. Available at:, R.R., 1993. Hypotheses on the Origins of Marine Larvae. Annual review of ecology and systematics, 24, pp.89–117. Available at:, C.M., 1990. Larval ecology of marine invertebrates: A sesquicentennial history. Ophelia, 32(1-2), pp.1–48. Available at: Heritage® Dictionary of the English Language, Fifth Edition. (2011). Accessed November 13 2015 at Sars, one of the Norwegian biologists giving the name to the Sars Centre, was among the first to describe the development of molluscs from a swimming larva (Sars, 1837; Young, 1990).

Read the previous section: Spiral cleavage, an oblique matter.

Annelids, arthropods and vertebrates show a remarkable morphological diversity (Chipman, 2010). Beneath this multiplicity of shapes and forms lies a common pattern of body organization—a trunk divided into repeated parts. This pattern and the developmental process that generates it are known as segmentation (Minelli and Fusco, 2004). While the vertebrate trunk is divided into somites1 (a portion of the mesoderm), the body of annelids and arthropods is divided into intricate repeated compartments spanning the ectoderm and mesoderm—the segments (Scholtz, 2002). The morphological similarity between these body segments previously was taken as support for a kinship between Annelida and Arthropoda, in a group called Articulata (Scholtz, 2002; Seaver, 2003). In this scenario, segmentation would have evolved only once in the protostomes and once in the deuterostomes (Davis and Patel, 1999; Peel and Akam, 2003; Seaver, 2003).Taxa with a segmented trunk. Annelida: the holoplanktonik polychaete Tomopteris sp., Arthropoda: a mantis shrimp (Stomatopoda), Vertebrata: a Teleostei fish larva. Yellow lines mark the anterior and posterior boundary of one segment. Image on the right is a closeup of the ectodermal segmentation of the fire worm Eurythoe complanata. Images not to scale. Photos by Alvaro E. Migotto (Migotto and Vellutini, 2011).Analyses arising from the area of molecular phylogenetics have disputed the monophyly of Articulata, suggesting that annelids and arthropods occupy different branches of protostomes, the Lophotrochozoa (=Spiralia) and Ecdysozoa, respectively (Aguinaldo et al., 1997; Eernisse, 1998). This phylogenetic hypothesis indicates that annelids and arthropods are more closely related to groups without body segmentation than to each other (Seaver, 2003); a topology that favors the independent evolution of annelid and arthropod body segmentation, in addition to the independent evolution of the different segmented tissues of vertebrates (Graham et al., 2014). Subsequent phylogenetic studies continue to corroborate the distant relationship between annelids, arthropods and vertebrates (Dunn et al., 2008; Dunn et al., 2014; Edgecombe et al., 2011; Hejnol et al., 2009), reinforcing the homoplasy of their body segmentation.Remarkably, the molecular mechanisms of body segmentation in arthropods and vertebrates show a number of striking similarities (Damen, 2007; Davis and Patel, 1999; Kimmel, 1996; Patel, 2003; Peel and Akam, 2003; Seaver, 2003; Tautz, 2004). These molecular similarities were taken as evidence to support the homology of bilaterian segmentation (De Robertis, 1997; De Robertis, 2008; Dray et al., 2010; Kimmel, 1996), despite the opposing data from phylogenetics. To reconcile this apparent conflict between developmental and phylogenetic data, we must apply a comprehensive evolutionary approach to the problem.The concept of segmentation is often used in a typological—and not evolutionary—manner (Budd, 2001). The result is a taxonomic bias, where the evolution of segmentation is regarded from the point of view of the groups considered to be segmented, i.e., annelids, arthropods and vertebrates (Budd, 2001). As a matter of fact, there is no conceptual basis to restrict segmentation to these three groups, because the repetition of parts along the body axis (Budd, 2001; Hannibal and Patel, 2013; Minelli and Fusco, 2004) also occurs in varying degrees in other bilaterians—usually considered to be pseudosegmented or unsegmented (Budd, 2001; Minelli and Fusco, 2004; Scholtz, 2002; Willmer, 1990).Another aspect to be considered is that segmentation—as much as spiral cleavage—is a complex of characters that ought to be individually compared between taxa (Scholtz, 2010). Breaking down segmentation into comparable traits (Scholtz, 2010), such as seriated nerve chords, segmented mesoderm or ectodermal boundaries, should provide a better overview of their evolutionary history.Nevertheless, the sole comparison of traits between distantly related groups can still be misleading for understanding the evolution of a character (e.g., trunk segmentation), because the ancestral conditions of closer taxa are unknown. Since developmental mechanisms can be coopted to nonhomologous structures (Shubin et al., 2009), the phylogenetic context of a character is essential to distinguish homology from convergence. A recurrent proposal to better understand the evolution of segmentation is to expand taxonomic sampling (Arthur et al., 1999; Budd, 2001; Couso, 2009; Davis and Patel, 1999; Minelli and Fusco, 2004; Patel, 2003; Peel and Akam, 2003; Seaver, 2003; Tautz, 2004). Thus, examining segmentation traits in a wider range of taxa, including those without obvious segmented features, might help us to grasp the evolution of the developmental mechanisms that form repeated body parts in bilaterians.

This text is the final section of my PhD thesis (published on this blog).

ReferencesAguinaldo, A.M. et al., 1997. Evidence for a clade of nematodes, arthropods and other moulting animals. Nature, 387(6632), pp.489–493. Available at:, W., Jowett, T. & Panchen, A., 1999. Segments, limbs, homology, and co-option. Evolution & development, 1, pp.74–76. Available at:, G.E., 2001. Why are arthropods segmented? Evolution & development, 3(5), pp.332–342. Available at:, A.D., 2010. Parallel evolution of segmentation by co-option of ancestral gene regulatory networks. BioEssays: news and reviews in molecular, cellular and developmental biology, 32(1), pp.60–70. Available at:, J.P., 2009. Segmentation, metamerism and the Cambrian explosion. The International journal of developmental biology, 53(8-10), pp.1305–1316. Available at:, W.G.M., 2007. Evolutionary conservation and divergence of the segmentation process in arthropods. Developmental dynamics: an official publication of the American Association of Anatomists, 236(6), pp.1379–1391. Available at:, G.K. & Patel, N.H., 1999. The origin and evolution of segmentation. Trends in cell biology, 9(12), pp.M68–72. Available at: Robertis, E.M., 1997. Evolutionary biology. The ancestry of segmentation. Nature, 387(6628), pp.25–26. Available at: Robertis, E.M., 2008. The molecular ancestry of segmentation mechanisms. Proceedings of the National Academy of Sciences of the United States of America, 105(43), pp.16411–16412. Available at:, N. et al., 2010. Hedgehog signaling regulates segment formation in the annelid Platynereis. Science, 329(5989), pp.339–342. Available at:, C.W. et al., 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature, 452(7188), pp.745–749. Available at:, C.W. et al., 2014. Animal Phylogeny and Its Evolutionary Implications. Annual review of ecology, evolution, and systematics, 45(1), pp.371–395. Available at:, G.D. et al., 2011. Higher-level metazoan relationships: recent progress and remaining questions. Organisms, diversity & evolution, 11(2), pp.151–172. Available at:, D.J., 1998. Arthropod and annelid relationships re-examined. In Arthropod Relationships. The Systematics Association Special Volume Series. Springer Netherlands, pp. 43–56. Available at:, A. et al., 2014. What can vertebrates tell us about segmentation? EvoDevo, 5(1), p.24. Available at:, R.L. & Patel, N.H., 2013. What is a segment? EvoDevo, 4(1), p.35. Available at:, A. et al., 2009. Assessing the root of bilaterian animals with scalable phylogenomic methods. Proceedings. Biological sciences / The Royal Society, 276(1677), pp.4261–4270. Available at:, C.B., 1996. Was Urbilateria segmented? Trends in genetics: TIG, 12(9), pp.329–331. Available at:, A.E. & Vellutini, B.C., 2011. Cifonauta – marine biology image database. Cifonauta, an image database for marine biology. Available at: [Accessed December 16, 2015].Minelli, A. & Fusco, G., 2004. Evo-devo perspectives on segmentation: model organisms, and beyond. Trends in ecology & evolution, 19(8), pp.423–429. Available at:, N.H., 2003. The ancestry of segmentation. Developmental cell, 5(1), pp.2–4. Available at:, A. & Akam, M., 2003. Evolution of segmentation: rolling back the clock. Current biology: CB, 13(18), pp.R708–10. Available at:, G., 2002. The Articulata hypothesis – or what is a segment? Organisms, diversity & evolution, 2(November 2001), pp.197–215. Available at:, G., 2010. Deconstructing morphology. Acta zoologica , 91(1), pp.44–63. Available at:, E.C., 2003. Segmentation: mono- or polyphyletic? The International journal of developmental biology, 47(7-8), pp.583–595. Available at:, N., Tabin, C. & Carroll, S.B., 2009. Deep homology and the origins of evolutionary novelty. Nature, 457(7231), pp.818–823. Available at:, D., 2004. Segmentation. Developmental cell, 7(3), pp.301–312. Available at:, P., 1990. Body divisions – metamerism and segmentation. In Invertebrate Relationships: Patterns in Animal Evolution. Cambridge University Press, pp. 39–45. Available at: addition to the somites, vertebrates also show segmentation in the rhombomeres and in the pharyngeal archs; segmented structures that likely evolved independently in the deuterostome lineage (Graham et al., 2014).

Read the previous section: What a larva is.

Francis M. Balfour set the pace on discussions about the evolutionary importance of larvae by addressing many of the fundamental questions regarding larval evolution (Balfour, 1874; Balfour, 1880; Balfour, 1881). He wondered about the ancestry of larvae. Can larvae reveal the ancestral forms of metazoans? He indicated tests to the predictions of recapitulation. Can we find a larva that corresponds to the adult of a related group? He asked whether larvae changed during evolution. How often do larval organs evolve? And what might be the underlying mechanisms for the evolution of development. What guides the maintenance or atrophy of larval organs in adult stages? (Hall and Wake, 1999).Perhaps, the greatest conceptual advance initiated by Balfour is that larvae are subject to variation and natural selection in the same manner as the adult stage (Balfour, 1874; Balfour, 1881). In other words, he articulated the realization that evolution can occur at any developmental stage. However, if not all embryonic features represent ancestors (or ancestral traits), the foundation of the recapitulation theory is compromised. The evolutionary debate caused by larvae influenced a more informed way to make extrapolations from ontogeny to phylogeny (Hall, 2000; Hall and Wake, 1999). It was no coincidence that one of the most vehement opponents of Haeckel’s recapitulation theory was a larvae affectionate, the biologist Walter Garstang who boldly concluded that “ontogeny does not recapitulate phylogeny, it creates it” (Garstang, 1922).Larvae of a brachiopod (left), a nemertean (center) and a bryozoan (right).Present-day research shows that larval traits are evolutionary labile, and often correlate to ecological, developmental and other life-history factors (Strathmann and Eernisse, 1994). Evidence from diverse taxa, including gastropods (Collin, 2004), sea urchins (Raff and Byrne, 2006), ascidians (Jeffery and Swalla, 1992), sea stars (Byrne, 2006; Hart et al., 1997), nemerteans (Maslakova and Hiebert, 2014) and polyclad flatworms (Rawlinson, 2014), indicates that larval forms were modified, gained or lost in different lineages independently, and that the observed similarities are likely the result of convergent evolution.These observations undermine scenarios about animal evolution that require the homology of larval characters (Jägersten, 1972; Nielsen, 1998; Nielsen, 2001; Nielsen, 2009; Peterson and Cameron, 1997) and are more consonant with the multiple independent evolution of metazoan larvae from a direct-developing ancestor (Page, 2009; Raff, 2008; Sly et al., 2003; Wray, 1995). Yet, the homology of larval characters such as the apical organ (e.g., Hunnekuhl and Akam, 2014; Marlow et al., 2014) or ciliated bands (e.g., Henry et al., 2007; Rouse, 1999) continues to be a central and lively discussed topic. For all the reasons above, larvae are a scandalous epitome of evolution, and the diversity of larval body patterns in marine invertebrates continue to provide a rich framework for evolutionary studies.

This text is a section of my PhD thesis. Read the next section: Spiral cleavage, an oblique matter.

ReferencesBalfour, F.M., 1874. Memoirs: A Preliminary Account of the Development of the Elasmobranch Fishes. The Quarterly journal of microscopical science. Available at:, F.M., 1880. A Treatise on Comparative Embryology, Macmillan and Company.Balfour, F.M., 1881. A Treatise on Comparative Embryology, Macmillan and Company. Available at:, M., 2006. Life history diversity and evolution in the Asterinidae. Integrative and comparative biology, 46(3), pp.243–254. Available at:, R., 2004. Phylogenetic effects, the loss of complex characters, and the evolution of development in calyptraeid gastropods. Evolution; international journal of organic evolution, 58(7), pp.1488–1502. Available at:, W., 1922. The Theory of Recapitulation: A Critical Re-statement of the Biogenetic Law. Journal of the Linnean Society of London, Zoology, 35(232), pp.81–101. Available at:, B.K., 2000. Balfour, Garstang and de Beer: The First Century of Evolutionary Embryology. American zoologist, 40(5), pp.718–728. Available at:;2.Hall, B.K. & Wake, M.H., 1999. Chapter 1 – Introduction: Larval Development, Evolution, and Ecology. In B. K. H. H. Wake, ed. The Origin and Evolution of Larval Forms. San Diego: Academic Press, pp. 1–19. Available at:, M.W., Byrne, M. & Smith, M.J., 1997. Molecular Phylogenetic Analysis of Life-History Evolution in Asterinid Starfish. Evolution; international journal of organic evolution, 51(6), pp.1848–1861. Available at:, J.Q. et al., 2007. Homology of ciliary bands in Spiralian Trochophores. Integrative and comparative biology, 47(6), pp.865–871. Available at:, V.S. & Akam, M., 2014. An anterior medial cell population with an apical-organ-like transcriptional profile that pioneers the central nervous system in the centipede Strigamia maritima. Developmental biology, 396(1), pp.136–149. Available at:, W.R. & Swalla, B.J., 1992. Evolution of alternate modes of development in ascidians. BioEssays: news and reviews in molecular, cellular and developmental biology, 14(4), pp.219–226. Available at:ägersten, G., 1972. Evolution of the Metazoan Life Cycle First Printing edition., Academic Press Inc.Marlow, H. et al., 2014. Larval body patterning and apical organs are conserved in animal evolution. BMC biology, 12(1), p.7. Available at:, S.A. & Hiebert, T.C., 2014. From trochophore to pilidium and back again – a larva’s journey. The International journal of developmental biology, 58(6-8), pp.585–591. Available at:, C., 1998. Origin and evolution of animal life cycles. Biological reviews of the Cambridge Philosophical Society, 73(02), pp.125–155. Available at:, C., 2001. Phylum Ectoprocta. In Animal Evolution: Interrelationships of the Living Phyla. Oxford University Press, pp. 244–263.Nielsen, C., 2009. How did indirect development with planktotrophic larvae evolve? The Biological bulletin, 216(3), pp.203–215. Available at:, L.R., 2009. Molluscan larvae: Pelagic juveniles or slowly metamorphosing larvae? The Biological bulletin, 216(3), pp.216–225. Available at:, K.J. & Cameron, R.A., 1997. Set-aside cells in maximal indirect development: Evolutionary and developmental significance. BioEssays: news and reviews in molecular, cellular and developmental biology, 19(7), pp.623–631. Available at:, R.A., 2008. Origins of the other metazoan body plans: the evolution of larval forms. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 363(1496), pp.1473–1479. Available at:, R.A. & Byrne, M., 2006. The active evolutionary lives of echinoderm larvae. Heredity, 97(3), pp.244–252. Available at:, K.A., 2014. The diversity, development and evolution of polyclad flatworm larvae. EvoDevo, 5(1), p.9. Available at:, G.W., 1999. Trochophore concepts: ciliary bands and the evolution of larvae in spiralian Metazoa. Biological journal of the Linnean Society. Linnean Society of London, 66(4), pp.411–464. Available at:, B.J., Snoke, M.S. & Raff, R.A., 2003. Who came first–larvae or adults? origins of bilaterian metazoan larvae. The International journal of developmental biology, 47(7-8), pp.623–632. Available at:, R.R. & Eernisse, D.J., 1994. What Molecular Phylogenies Tell Us about the Evolution of Larval Forms. Integrative and comparative biology, 34(4), pp.502–512. Available at:, G.A., 1995. Punctuated evolution of embryos. Science, 267(5201), pp.1115–1116. Available at:

Read the previous section: Larvae as the epitome of evolution.

By the end of the 19th century, a series of biologists had dedicated themselves to following and discovering the fate of individual cells of an embryo during ontogeny. These works, known as cell lineage studies1, were critical to disambiguate the relationship between ontogeny and phylogeny, directly challenging the idea of recapitulation (Guralnick, 2002; Maienschein, 1978).The detailed work of the cell lineage biologists Edmund B. Wilson, Edwin G. Conklin, Frank R. Lillie and others, revealed something remarkable. After carefully tracing the embryonic cells of different organisms, they discovered that animals such as molluscs, annelids, nemerteans and polyclad flatworms, whose adult stages are so different, actually share a similar embryogenesis2 (Child, 1900; Conklin, 1897; Heath, 1899; Lillie, 1895; Mead, 1897; Wilson, 1892). Their embryos show the same cleavage pattern, in which cell divisions occur with the mitotic spindles oblique to the animal/vegetal axis, switching direction (clockwise and counterclockwise) at each division cycle (Costello and Henley, 1976; Hejnol, 2010; Henry and Martindale, 1999; Lambert, 2010). A quartet of vegetal macromeres sequentially gives rise to animal micromeres, and the resulting symmetry of these cleaving blastomeres, when viewed from the animal pole, was described as spiral. This developmental pattern thus became known as spiral cleavage (Wilson, 1892).The spiral cleavage pattern. (A) Animal pole view of a generalized spiral-cleaving embryo. Arrows indicate the direction of cell divisions. Developmental sequence based on (Conklin, 1897). (B) Schematic diagram of cell divisions in the D quadrant in a lateral view (top: animal pole, bottom: vegetal pole). Cells are named with the standard spiral cleavage notation(Child, 1900; Conklin, 1897; Wilson, 1892). Representation based on Lambert (2010).Because the cell divisions are stereotypic, individual blastomeres can be followed and compared between spiral-cleaving taxa in a fairly consistent manner. The ability to compare blastomere fates at this unprecedented cellular-resolution uncovered a surprising similarity in the fate maps of spiral-cleaving embryos (=annelids, molluscs, nemerteans and polyclad flatworms). The iconic example being the 4d mesentoblast, a well-conserved mesoderm precursor (Lambert, 2008). Overall, despite having the oblique cell divisions as an idiosyncrasy, spiral cleavage is understood today as a complex of developmental characters (Costello and Henley, 1976; Hejnol, 2010; Henry and Martindale, 1999; Lambert, 2010).The empirical findings of cell lineage studies raised several important evolutionary questions regarding the evolution of development and the establishment of homologies (Guralnick, 2002). What are the underlying causes behind embryonic cleavage patterns—mechanical forces acting on the embryo or inherited historical factors? Are the events of early development necessary to build the adult characters? Is there an embryological criterion for homology? The ideas progressively moved towards a more evolutionary view of development, where ontogeny is not “a brief and rapid recapitulation of phylogeny” but an inherited product of evolution and subject to modification (Guralnick, 2002).Even though most cell lineage biologists initially denied the systematic value of embryonic cleavage patterns, mainly in opposition to recapitulation (Guralnick, 2002), it was difficult to argue against the striking similarity between spiral-cleaving embryos, and dismiss their potential kinship3. Schleip (1929) was the first to propose a group to contain the animals displaying spiral cleavage—the Spiralia.Recent metazoan-wide phylogenetic analyses corroborate the kinship between spiral-cleaving taxa, in a major protostome clade that is sister to the Ecdysozoa (e.g., insects) (Dunn et al., 2014). The latest works in protostome phylogenomics (Laumer et al., 2015; Struck et al., 2014) suggest that Spiralia (=Lophotrochozoa in some cases, see Hejnol (2010)) contains not only the typical spiral-cleaving groups, but several other taxa. Some spiralians (=animals that belong to the clade Spiralia) do not show any clear trace of spiral cleavage, such as bryozoans, brachiopods, gastrotrichs and rotifers, while others do exhibit spiral-like characters, such as gnathostomulids (Riedl, 1969), phoronids (Pennerstorfer and Scholtz, 2012) and entoprocts (Marcus, 1939; Merkel et al., 2012). What can we say about the evolution of these disparate cleavage patterns?The spiral arrangement of embryonic blastomeres is present in the three main clades of Spiralia (Gnathifera, Lophotrochozoa and Rouphozoa), suggesting that this character is ancestral at least to the Lophotrochozoa-Rouphozoa clade. This implies the spiral cleavage pattern was lost during the evolution of gastrotrichs, brachiopods, bryozoans and maybe rotifers. How did these groups lose spiral cleavage? Which aspects of a typical spiral-cleaving embryo did they lose, in addition to the spiral arrangement of the blastomeres? Are there any remnants of spiral cleavage?The comparison between clades that have lost spiral symmetry, like bryozoans and brachiopods, and typical spiral-cleaving clades such as annelids and molluscs, can identify the traits that were lost, or are still shared, among these groups. This comparative approach can reveal novel insights about the evolution of spiral cleavage itself, and give rise to a broader perspective of the evolutionary mechanisms underlying spiralian development.

This text is a section of my PhD thesis. Read the next section: Segmentation, a question of boundaries.

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