Malacostracan: Periclimenes

Ubx ab+d-A

shrimp Artemia, is quite different from Drosophila and other insects. In Artemia, the anterior boundary of Ubx falls at the anterior of the thorax, at the transition from the gnathal head segments (which bear feeding appendages) to the thoracic segments (which bear swimming appendages). Thus the expression of this Hox gene marks a transition in appendage morphology along the Artemia anteroposterior axis, but Ubx is expressed at a more anterior position relative to the insects.

Other, more derived crustacean lineages that possess specialized thoracic limbs exhibit different anterior boundaries of Ubx expression (Fig. 5.3). In some malacostracan and maxil-lopod crustacean lineages, one to three pairs of thoracic limbs are reduced in size and are used as feeding appendages (maxillipeds). In these organisms, the anterior boundary of Ubx/abd-A protein expression consistently lies to the posterior of any maxilliped-bearing thoracic segments. Interestingly, in some species, the loss of Ubx/abd-A expression in anterior thoracic limbs precedes the developmental transition from a larger limb morphology early in embryogenesis to a smaller maxilliped later in development. In these crustaceans, a shift in Hox gene expression anticipates the formation of specialized thoracic limbs.

The myriapods exhibit a body organization that differs from the body organizations of both the insects and crustaceans, and the boundary of Ubx protein expression occurs at a different segmental position (Fig. 5.2). In the centipede, the anterior boundary of Ubx/abd-A expression lies within the second trunk segment and extends through much of the homonomous trunk. Again, this boundary of Hox expression marks a transition in limb identity. Early in development, no Ubx protein is expressed in the developing poison claw on the first trunk segment, whereas it is expressed in all of the developing walking legs on the remaining trunk segments. In the fourth living arthropod class, the chelicerates, the anterior boundaries of Ubx and abd-A expression fall within the limbless opisthosomal segments (Fig. 5.2).

In summary, the relative positions of the anterior boundaries of Ubx and abd-A expression have shifted between the various arthropod classes, and even within orders of crustaceans. These shifts correlate with differences in body organization—in particular, with the morphology of thoracic limbs in insects, crustaceans, and myriapods.

In contrast to the highly diversified and patterned segments of arthropods, the ony-chophoran body plan includes fewer distinct segmental identities and a homonomous trunk. Where are the onychophoran orthologs of the trunk Hox genes Ubx and abd-A expressed? One might have guessed that their deployment would be similar to the arthropods, in which these Hox genes are expressed in most of the trunk segments. In reality, Ubx/abd-A protein expression is limited to the extreme posterior of the onychophoran embryo, in the last pair of lobopods and in the terminus (Fig. 5.2). Although Hox genes are conserved between onychophora and arthropods, clearly their expression patterns have changed significantly during their independent evolutionary history.

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    Do annelids have UBX and abdA hox genes?
    8 years ago

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