Euaresta Main | Tephritidae Main | Diptera Home | SEL Home

Euaresta Phylogeny

Norrbom (1993) analyzed the relationships among all but one of the species of Euaresta (E. stelligera was not included, but is presumably closely related to E. bellula and E. jonesi). The character matrix lists the 13 characters used in that analysis, and the distributions of their states. The matrix has been slightly modified from that in Norrbom (1993) in that character states are also shown for E. stelligera, and the state of character 13 in E. bellula has been corrected.

The following character states are considered autapomorphies for individual species and were not included in the analysis: the broad, orange frons, the dark legs and abdomen, the straight margin of male sternite 5, and the unusual male chaetotaxy and microtrichial patterns in E. versicolor; the nonmicrotrichose male sternite 5 in E. toba; the elongate oviscape in E. regularis; the reduced eye size, and the orange oviscape in E. aequalis; the arista color and shape, and the male antenna color in E. bullans; and the bicolored wing pattern, and the even distribution and small size of the reticulations in the proximal 2/3 of the wing in E. reticulata.

Euaresta belongs to the tribe Tephritini (Foote et al. 1993), but its exact relationships within the tribe remain unclear. Because its sister group is uncertain, representative species of all other New World genera of Tephritini were examined to determine character polarities by outgroup comparison. For characters for which only one state occurs in all other Tephritini (i.e., characters 1, 2, 3, 5, 10, 13), polarities within Euaresta were easily hypothesized. For characters that vary among other Tephritini (e.g., characters 6, 7, 9 and 12), where possible the polarity that makes the character state distribution most congruent with those of characters of unequivocal polarity was hypothesized. For example, with character 7 polarized as in Table 1, the distribution of its states is congruent with that of character 5; reversing its polarity would cause homoplasy in regard to character 5. For these characters, it is interesting to note that the state coded plesiomorphic by the above method also is the more common state among other Tephritini, although this does not mean that the common equals primitive method of determining character polarity was used to support the character analysis. The polarities of several characters (4, 8, 11) could not be determined. These were coded with alternate states in the outgroup in a variety of preliminary analyses with little effect on the resulting trees (e.g., two steps are required for character 8 no matter which polarity is hypothesized for it because in both cases it is incongruent with several other characters). These are further discussed below.

Analysis of the matrix in Table 2 of Norrbom (1993) by the implicit enumeration option (ie*) of Hennig86 resulted in four trees of 20 steps (consistency index = 0.80, retention index = 0.88). Successive weighting did not reduce the number of trees. The original trees differ from the Nelson concensus tree in having either E. aequalis, E. reticulata, or both grouped with E. bullans, E. philodema, and E. meridionalis, depending upon the interpretation of characters #6 and #7. The Nelson tree therefore is one step longer (length = 21 steps, consistency index = 0.76, retention index = 0.86).

The following are the significant results of the analysis. The broad epandrium (character #1) is a synapomorphy for Euaresta. This state does not occur in any other Tephritini. The swollen male fore femur (#2) is probably another synapomorphy of the genus, with reversal to the plesiomorphic state in E. versicolor, but it also may be interpreted as a synapomorphy for E. toba + E. regularis, and another for the bullans group.

Euaresta includes two monophyletic groups, the toba group and the bullans group. The toba group includes E. toba, E. regularis, and E. versicolor, for which the loss of the anterior notopleural seta (#3) and the reduced apical scutellar seta (#4) are synapomorphies. The relationships among E. toba, E. regularis, and E. versicolor are poorly resolved. The closer relationship of E. toba and E. regularis is weakly supported only by character #11, which is difficult to analyze (see below) and by one interpretation of character #2 (see above). Most characters that differentiate these three species are autapomorphies for one of them.

The bullans group includes all of the species of Euaresta other than E. toba, E. regularis, and E. versicolor. Its monophyly is supported by the shape of the epandrium, which is flattened or concave posteriorly, with strong ridges or striations (#5). Within the bullans group there are two monophyletic subgroups: E. philodema, E. bullans, and E. meridionalis form the bullans subgroup; and E. tapetis, E. festiva, E. bella, E. stigmatica, E. bellula, E. stelligera and E. jonesi comprise the bella subgroup. As stated above, E. reticulata is grouped most closely with the bullans subgroup by character #6, whereas E. aequalis is placed in this position by character #7.

The relationship of E. tapetis with the rest of the bella subgroup is based only on character state #12.1, which may not be reliable because the presence of a weak bulla in cell r₄₊₅ is intraspecifically variable in E. tapetis. Also, if the polarity is reversed for character #11, which was difficult to code and polarize, some of the resulting trees group E. tapetis with the bullans subgroup, E. aequalis, and E. reticulata. A Hennig86 analysis of the matrix with the polarity reversed for character #11 produced 16 trees of 21 steps (consistency index = 0.76, retention index = 0.86). Except for sometimes placing E. tapetis with the bullans subgroup, they otherwise are similar to the trees of the first analysis except that the three species of the toba group sometimes form an unresolved trichotomy.

The hypotheses of phylogenetic relationships within Euaresta discussed above permit limited analysis of the biogeography of the genus. It should be noted that the introduction of E. bullans to California was presumably by man, sometime prior to 1903 when the types of E. adspersa Coquillett (= bullans) were collected. If the sister group of Euaresta is Neotropical, it is most parsimonious to assume that the genus originated in that region with two dispersals to North America, by the ancestor of the bella subgroup, and by E. aequalis or its ancestor. If the sister group is Nearctic or occurs in both regions, that hypothesis is equally likely to the following, that Euaresta originated in North America, with dispersals to South America by the ancestors of the toba group and bullans subgroup. Another dispersal event would be required if E. aequalis, rather than E. reticulata, is the sister group of the bullans subgroup.

The fact that all of the known hosts for E. toba, E. regularis, and the species of the bella subgroup are Ambrosia suggests that species of this plant genus may have been the original hosts of the common ancestor of Euaresta. Interesting questions that remain to be answered include whether all of the species that breed in Xanthium form a monophyletic group, and what the hosts are of the other species. If there has been only one shift from Ambrosia to Xanthium by Euaresta, the cladogram suggests that E. versicolor breeds in Ambrosia, and that E. meridionalis breeds in Xanthium. If E. reticulata is more closely related to the bullans subgroup than is E. aequalis, it probably also breeds in Xanthium, although if E. aequalis is closer, no host prediction can be made from the cladogram.