Author: Grace Birkett
Editor: Altay Shaw
In scientific research, most model organisms are direct developers, with postembryonic stages representing a smaller version of the adult that grows by stages of moulting. However, the most abundant developmental mode across the Metazoa is the indirect life cycle, which involves a larval stage that occupies a distinct ecological niche from the adult form. The most familiar indirect developers belong to the tadpole larvae of amphibians and tunicates, or the caterpillar larvae of lepidoptera. The complexities of these larvae, known as secondary larvae, include multilayered tissues and a central nervous system (CNS). Other, often overlooked indirect developers belong to the Porifera, cnidarians, echinoderms, hemichordates, and most of the Spiralia. These bear simple pelagic larval forms, described as primary larvae, that metamorphose into large-bodied, often benthic adults. Some of these larvae feed (planktotrophic), whereas others rely on energy reserves provided by the maternal yolk (lecithotrophic).
The primary larvae are extremely diverse in morphology. The early Metazoa – Porifera and cnidarians, have the simplest larval structures. The planula larvae of cnidarians are typically flattened and circular, while the ciliated surfaces of Porifera larvae are similar. As for the echinoderms, there are two main types of larva: the pluteus and dipleurula. Pluteus larvae have skeletal rods that support outward-extending arms. However, dipleurula larva present no skeleton but rather broad lobes of tissue. They have two ciliary bands: an anterior loop surrounding the front of their body and a posterior loop encircling the anal field. These bands sometimes merge to form a single, continuous band. The echinoderm’s sister group, the hemichordates, bear tornaria larva. These have an anal ciliated ring—the telotroch, which bears long cilia that rotate during swimming. In addition to the telotroch, there are ciliary bands around the mouth that are used to push food into the mouth. Also, the Spiralia includes many different phyla with unique larval forms, such as molluscs, annelids, ectoprocts, nemerteans, and platyhelminthes. More specifically, it includes phoronids. Platyhelminthes like Polycladida have three larval types (Muller’s, Kato’s, and Goette’s), each differing in lobe number and eye arrangement. Nemerteans possess planuliform or pilidium larvae, while phoronids have actinotroch larvae with tentacles for feeding and swimming. Brachiopods exhibit two larval forms with different lobes and shells, and ectoprocts have both planktotrophic and lecithotrophic larvae. Molluscs and annelids both have a trochophore larva, which looks a lot like some larvae from other spiralian phyla. This suggests that these groups may have evolved together.
Despite this diversity, there are numerous morphological similarities in primary larvae. For example, all primary larvae bear an apical organ—a sensory structure equipped with serotonergic neurones and an apical tuft. The apical organ’s function remains elusive, but some studies have suggested its involvement in locomotion and sensing the correct environmental cues to initiate metamorphosis. As mentioned, most primary larvae have bands of cilia, which enable further detection of the environment and aid in swimming. Such similarities have introduced the possibility of a single origin of larval forms as opposed to convergent evolution due to common selection pressures. In fact, recent studies have gone beyond comparative morphology and analyzed gene expression patterns to determine the true homologies of these structures. The molecular topography of the apical organ has been well characterised in numerous metazoa, notably the Cnidaria, annelids, and the deuterostomes which are collectively referred to as the Neuralia. Comparing gene expression across the Neuralia was mainly done by in situ hybridisation (ISH). Fascinatingly, researchers found that genes were expressed in a concentric ring pattern, including foxj, nkx3, hox1, and irx. This could represent a larval-specific genetic signature that defines the apical organ and could imply a single evolutionary origin of larvae.
However, as for the ciliary bands, the evolutionary story is not so clear. Using single-cell sequencing of Muller’s larvae from a tiger flatworm and the trochophore larvae of Pacific oysters, one study characterised ciliary bands. Coexpression of genes between ciliary clusters was identified, and not surprisingly, most of them encode proteins for the ciliary apparatus. Researchers also identified specific transcription factors (TFs) like otx in the ciliary bands, which were previously known for their conserved functions across various organisms such as other molluscs, annelids, hemichordates, and echinoderms. However, extension to pluteus larvae revealed some gene co-expression (mainly in the gut), yet no shared TFs and very few similarities between ciliary bands. Considering these findings as they stand, they suggest strict conservation within Spiralia, with protostomes and deuterostomes independently evolving their own ciliary bands.
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