Abstract
BackgroundThe development of specialized appendages involved in sperm transfer in the males of julid millipedes is an extreme case of specialized, complex structures differentiating in a very advanced phase of post-embryonic development. Here, a non-systemic metamorphosis affects the external morphology and the internal anatomy of a trunk double segment only.Presentation of the hypothesisWe hypothesize that during early (possibly embryonic) development a segmental marker is produced that remains unexploited throughout late embryonic and early post-embryonic development, until, activated by a systemic signal, it finally determines the release of a segmentally localized but anatomically major change.Testing the hypothesisKey to testing the hypothesis are (1) the identification of both the putative segmental marker involved in the localization of the legs to be eventually metamorphosed into gonopods and the systemic signal activating it, (2) the identification of the cell population from which the gonopods are built, and (3) a longitudinal study of the marker's expression throughout late embryonic and, possibly, post-embryonic development.Implications of the hypothesisProving the validity of this hypothesis would demonstrate the existence of a cryptic developmental module that will be activated only months, or years, after it has been first laid down during early development. This study also opens a window onto the very poorly explored domain of late expression of developmental genes and molecular control of late developmental events.
Highlights
The development of specialized appendages involved in sperm transfer in the males of julid millipedes is an extreme case of specialized, complex structures differentiating in a very advanced phase of post-embryonic development
The development of specialized appendages involved in sperm transfer in the males of julid millipedes (Diplopoda, Julida) [FIG. 1] is an extreme case of specialized, complex and highly species-specific structures differentiating in a very advanced phase of post-embryonic development. This occurs through a metamorphosis that deeply affects external morphology and internal anatomy of the trunk diplosegment bearing the eighth and ninth pair of legs, while leaving unaffected the sections of the trunk that both precede and follow it
We propose to call this kind of dramatic post-embryonic transformations confined to a circumscribed body district non-systemic metamorphosis
Summary
Translation of positional information into the localized expression of anatomical structures is not limited to the embryonic phase of development, or to a metamorphosis systemically affecting the whole animal body. 1] is an extreme case of specialized, complex and highly species-specific structures differentiating in a very advanced phase of post-embryonic development This occurs through a metamorphosis that deeply affects external morphology and internal anatomy of the trunk diplosegment bearing the eighth and ninth pair of legs, while leaving unaffected the sections of the trunk that both precede and follow it. Proving the validity of this hypothesis would demonstrate the existence of a cryptic developmental module that will be activated only months, or years, after it has been first laid down during early development and the existence of positional markers whose information can even be used repeatedly (periodomorphosis), following morphogenetic reversals of a magnitude and significance only comparable to the loss and regeneration of major body parts In evolutionary perspective, this change cannot be equated to the change from the thoracic legs of a caterpillar to the thoracic legs of a butterfly, because in the latter case it is the adult appendage which is phylogenetically conservative, while the larval leg is evolutionarily more derived. Gonopod evolution in millipedes possibly represent a case of developmental constraints, whose precise nature needs be investigated at molecular level
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