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7.1A). The expression of Tc ser was compared to the expression of the PD domain Fig.7.1 Evolutionary implications of the serial homology of the cardo to the subcoxa of the mandible. Schematic of evolution of the mandible and maxilla from an identical serially homologous limb. Two evolutionary paths from a common serially homologous limb are illustrated, the mandible and maxillary evolutionary paths. The coxa (cx) and basis (ba) are indicated. The telopodite is shown in blue and the protopodite is shown in yellow. Dotted line indicates embryological subcoxa/coxa division that does not result in a segment boundary. If the subcoxa (red segment) is homologous between the mandible and maxilla, it implies that the ancestral protopodite consisted of three segments. Serially homologous endites present on the coxa are indicated with an arrowhead. (A) Hypothesized ancestral biramous appendage from which the mandible and maxilla evolve. Protopodite has subcoxa, coxa and basis. (B) Hypothesized ancestral mandible, based on Cambrian crustacean Bredocaris mandibular limb morphology. (C) Hypothesized ancestral mandible. (D) Crustacean mandible with telopodite palp present and hypothetical division into subcoxa and coxa. (E) Unsegmented Insect mandible with hypothetical subcoxal/coxal division.
genes in order to identify each appendage segment. There are significant similarities of PD domain gene expression that are suggestive of serially homologous relationships between the subcoxa of the mandible, maxilla and labial appendages.
By comparing endites together with their segmental affinities it is possible to demonstrate homologous relationships. The insect maxilla, consisting of two segments (cardo and stipes), has probably evolved from a three segmented archaeognathan maxilla (subcoxa, coxa and basis) (compare fig. 7.1I and 7.1J) The molar and incisor processes derived from an endite present on the mandibular coxa (see fig. 7.1E and 7.2D). If the maxillary coxa of archaeognathans (see fig.7.1I) is homologous to the mandibular coxa, it suggests that the mandibular endite is homologous to the lacinea endite which is present on the coxa of the maxilla.
Serial homology of the subcoxa and coxa of the mandible and legs Boxshall has commented that if the pleuron of the leg has a subcoxal origin then it is possible to serially homologize the leg protopodite segments to other appendages (Boxshall, 2004). The expression of Tc ser indicates that there is a subcoxa segment in the leg that develops into the pleuron in the larva confirming the subcoxal origin of the pleuron. Therefore there is a possibility that this putative subcoxa segment can be serially homologized to protopodite segments of the gnathal appendages. An evolutionary scenario which illustrates the serial homology of the subcoxa from a primitive three segmented protopodite to the insect mandible, maxilla and leg is illustrated in fig.7.2.
There are numerous similarities between the developing subcoxa and coxa of the Tribolium leg to the subcoxa and coxa of the gnathal appendages that suggest serial homology. The coxal-2 domain of Tc ser expression is activated at the same time in the maxilla, labial and leg limb buds. The proximal domain of Tc dac is activated with the coxal-2 domain of Tc ser. The subcoxal-1 domain of Tc ser is activated at the same time in all post-antennal appendages. There are also similarities of the maxilla telopodite to the developing leg. For example, in the leg and maxilla, Tc Dll and Tc hth are co-expressed with the trochanteral-3 Tc ser domain of Tc hth.
Accepting all these similarities of gene expression as evidence of serial homology, the leg coxa would be serially homologous to the maxillary stipes, the mandibular coxa and to the labial prementum. The leg subcoxa would be serially homologous to the maxillary cardo, the mandibular subcoxa and the labial postmentum.
However, there are some subtle differences between the proximal domains of Tc dac of the leg appendages compared to the gnathal appendages. These differences may reflect fundamentally different patterning mechanisms of these coxal segments, or may result from the different morphology of the limbs based on the presence of endites in the gnathal appendages. There are in fact three domains of expression of Tc Fig.7.2 Serial homology of the mandibulary subcoxa to the maxillary cardo (subcoxa) and leg pleuron (subcoxa) of insects. Serial homology of the subcoxa of the insect mandible (D), maxilla (F) and leg (H) requires that the subcoxa is derived from the same proximal segment on the ancestral biramous limb (A). In this scheme, the insect appendages (D,F,H) evolved through primitive precursor appendages present on a hypothetical pancrustacean ancestor (C,E,G). This scheme homologises the mandibular endite to a single endite present on the coxa segment of the maxilla (arrowhead). Three evolutionary paths of three beetle appendages (mandible, maxilla and leg) from an ancestral biramous limb (A). (A) Hypothetical primitive post-antennulary limb with a three segmented protopodite on a stem lineage mandibulate based on limb of Martinssonia elongata. (B-D) mandible evolution. (B) Hypothetical stem lineage mandibulate arthropod mandible with two palps and mandible with hypothetical subcoxal/coxal division. (C) hypothetical pancrustacean mandible mandible with hypothetical subcoxal/coxal division and a palp.
(D) Insect dicondylic mandible with hypothetical subcoxal/coxal division. (E-F) maxilla evolution. (E) hypothetical primitive pancrustacean maxillule. (F) insect maxilla with fused coxa and basis segments. (G-H) leg evolution. (G) hypothetical pancrustacean primitive biramous limb. (H) Insect leg with subcoxa derived pleuron. Dotted line indicates hypothetical embryological subcoxa/coxa division that does not result in a segment boundary in larval or adult forms. If the subcoxa (red segment) is homologous between the mandible and maxilla it implies that the ancestral protopodite consisted of three segments: subcoxa, coxa and basis. If the subcoxa of the insect leg (H) is homologous to the subcoxa of the insect maxilla (F) and mandible (D) it requires that these segments were present in the evolutionary lineages leading to all this limbs from an ancestral limb with a subcoxa present (A).
dac in the developing leg (Prpic et al., 2001), a distal domain in the femur, a larger proximal domain which overlaps with the coxal-2 domain of Tc ser and another smaller spot of expression which is co-expressed with the subcoxal-1 domain of Tc ser (data not shown). The proximal domain of Tc dac expression in the leg is co-expressed with the coxal-2 domain of ser expression throughout development. This is in contrast to Tc dac expression in the gnathal appendages where the proximal expression domain is expressed in between the coxal-2 and subcoxal-1 domains of Tc ser expression domains in the developing coxa. This difference could be because of the presence of endites in the coxa of the gnathal appendages which are lacking in the leg coxa.
Serial homology of the subcoxa of the mandible to the subcoxa of the maxilla Arthropod post-antennal appendages evolved from serially homologous biramous limbs. At some point in the evolution of the mandible and maxilla, the limbs were identical in structure and then diverged to evolve into the myriad forms of mandibles and maxillae that are present today (see fig.7.1).
One reasonable account of mandible and maxilla evolution posits that the ancestral protopodite on these appendages consisted of a two segmented protopodite, the coxa and basis. There were likely to be multiple endites present as this is commonly found in stem lineage crustaceans like Martinssonia and Phosphatocopida and stem lineage branchiopods like Rehbachiella (Muller and Waloszek, 1986; Waloszek, 1993; Siveter et al., 2001). The mandibles of myriapods, insects and some (especially terrestrial) crustaceans have lost both palps. The gnathal edge of the mandible is considered to be present on the proximal protopodite segment, which is the coxa (see fig 1.5), as the majority of mandibles do not have a subcoxal segment.
The serial homology of the subcoxa of the mandible and maxilla would suggest that the primitive mandible and maxilla originally had three protopodite segments, the subcoxa, coxa, and basis (fig. 7.1A). Maxillary protopodites with three segments are present in crustaceans (fig. 7.1H) and archaeognathan hexapods (fig.7.1I).
Evolution of the insect mandible from an ancestral maxilla-like precursor with a three segmented protopodite (see fig. 7.1A) could occur as follows (see fig.1.A-G): The proximal endite is modified to form the mandible gnathal edge (fig. 7.1B). The basis is reduced in size (fig.7.1C). The exopodite palp is lost, the basis is further reduced in size and the subcoxa fuses with the coxa (fig.7.1D). Finally, the telopodite palp is lost (fig.7.1E).
The subcoxa and coxa fuse such that external evidence of segmentation is lost and form the coxa which is evident in mandibles of the majority of living mandibulates.
Study of the embryological development of the Tribolium mandible, as shown in chapter three, however reveals evidence of segmentation in the form of a subcoxacoxa boundary and a subcoxal Tc ser domain of expression.
Hypothesized evolution of a maxilla protopodite with three segments is shown in fig. 7.1. Evolution from a three segmented maxilla protopodite does not require addition of segments in different lineages, but rather would be typified by loss and fusion of segments, for example, the insect maxilla has evolved by the fusion of the coxa and basis to form the stipes (fig. 7.1J).
If the subcoxa of the mandible and maxilla of Tribolium are not serially homologous, the similarity of Tc ser expression in the protopodite requires an explanation. It is difficult to speculate without knowing the precise functions of the proximal domains of PD domain genes but the similarity of the first two ring expression domains of Tc ser could reflect similarities of the gene regulatory network of limb patterning that has nothing to do with a serial homologous relationship. The gene regulatory network could have evolved in the lineage leading to Tribolium such that the similarities observed between appendages merely reflect the similarity of the patterning mechanism for limbs in general.
Criticism of serial homology hypotheses A trend in evolutionary development is to make inferences of serial homology between appendages based upon data obtained from one species. The reasons for this are understandable, as it is often difficult enough to study one organism in a laboratory let alone the several required to make strongly supported inferences about ancestral character states. Machida’s hypothesis of serial homology between the mandible and maxilla is based upon evidence from one species of bristletail (Machida, 2000). Prpic’s hypothesis of the serial homology of the complete mandible to the coxa was based solely upon the expression of the proximal domain of Tc dac in Tribolium (Prpic et al., 2001). The present study is also susceptible to this criticism, by comparing the expression of Tc ser and the PD domain genes in different appendages.
It is easy to overinterpret apparent similarities of expression from one organism without providing the proper phylogenetic context of the evolution of these limbs.
However, the combination of such diversity of appendage forms, the 100 million year ghost lineage of myriapod fossils and the difficulty of interpreting Cambrian arthropod appendage segments makes it difficult to infer the likely primitive character state of the ancestral mandible.
Certain key taxonomic relationships are still lacking, which would help in understanding the polarity of character states, that is to say, whether they are primitive or derived. There is little consensus about the sister group to the hexapods within Pancrustacea (Edgecombe, 2010; Jenner, 2010).