User:Boccardia/sandbox

Poecilogony (from the Greek poikilos (ποικίλος), meaning "various"; and gony (γόνος), meaning "offspring") is a rare phenomenon in which a single species of marine invertebrate has larvae with multiple modes of development. The term was first coined by French zoologist Alfred Mathieu Giard in 1891, though it is his 1905 paper that is most widely cited. The existence of poecilogony has been somewhat controversial since many species previously thought to be poecilogonous have turned out to be cryptic species. As such, poecilogonous species are largely restricted to two main groups of marine invertebrates: the opisthobranch molluscs and the spionid polychaetes (Annelida). Divergent larval phenotypes in poecilogonous species may represent genetic polymorphisms or environmentally-induced polyphenisms (or some combination of the two). In the latter case, poecilogony may be considered a special form of phenotypic (or developmental) plasticity.

Examples of Poecilogony
As stated, poecilogonous species usually belong to the opisthobranch molluscs or spionid polychaetes. Examples of poecilogonous opisthobranchs include Elysia chlorotica, Elysia zuleicae, Alderia willowi, and Costasiella ocellifera. Examples of poecilogonous spionids include Streblospio benedicti, Boccardia proboscidea , Pygospio elegans , and Polydora cornuta.

Divergent Traits and Common Features
In poecilogonous species, larvae are typically divergent in one or more of the following traits:

Divergence in larval traits affects several life history consequences, including but not limited to:

Most poecilogonous species share a few features in common, including:
 * brood protection
 * maternal provisioning (e.g., nurse eggs)

Controversy
There remains some controversy over the existence of true poecilogony, since many species previously thought to be poecilogonous have turned out to be cryptic species. A 1988 review by Hoagland and Robertson proposed that at least one of the following types of data must exist to support the hypothesis of poecilogony:
 * 1) molecular analysis (e.g., cytochrome c oxidase subunit I (CO1) analyses)
 * 2) observations of a single individual producing two or more types of larvae under controlled conditions
 * 3) interfertility analysis

Mechanisms
There are several potential mechanisms by which poecilogony is thought to occur:
 * Genetic polymorphism
 * For example, divergence in Streblospio benedicti has a genetic basis whereby dichotomies between planktotrophic and lecithotrophic larvae have high genetic correlations.


 * Environmentally-induced polyphenism
 * For example, Alderia willowi and certain populations of Polydora cornuta likely represent environmentally-induced polyphenisms whereby environmental cues determine development mode. Recent research suggests that epigenetic mechanisms (such as histone modifications) that are susceptible to environmental perturbation may underlie developmental plasticity in species as plastic as Polydora cornuta.


 * Maternal effects (e.g., extra-embryonic provisioning)
 * Studies in the lab indicate that prematurely rupturing the egg capsules of certain species can cause lecithotrophic young to become planktotrophic (i.e., they exhibit "facultative planktotrophy"). Maternal provisioning--in the form of nurse eggs, yolk strings, or egg size--therefore, plays a key role in determining larval mode of development.

Divergence in larval phenotype may also represent some combination of the above. For example, females of the poecilogonous species Boccardia proboscidea produce both adelphophagic and planktotrophic larvae. At least in the North America West Coast population, female B. proboscidea from higher latitudes brood offspring with more nurse eggs per capsule than females from southern latitudes. Therefore, species such as Boccardia proboscidea likely represent intermediates in which genetic and environmental factors both contribute to the maintenance of multiple development modes.

Value in Studying Poecilogony
The value in studying poecilogony, though rare among marine invertebrates, is that it allows us to understand evolutionary transitions in larval phenotype and development.