Política social en Venezuela desde los años setenta

Do host races exist ?

Yes. By far the m ost parsimonious interpretation of the Rhagoletis pomonella data is that apple and haw thorn associated populations are host races. Replicable genetic

differentiation betw een the two has been exhaustively dem onstrated, and there is very strong direct evidence that the level of actual gene flow betw een the tw o populations is approxim ately 6% in areas of overlap.

Similar evidence, discussed throughout the rem ainder of this thesis, suggests that

Zeiraphera diniana on larch and pine are also host races, although in this case the direct estimate of gene flow is lower, approximately 2.4% per generation (Chapter 2). Eurosta solidaginis on Solidago altissima and S. gigantea, are also very likely to be host races. Again, genetic differentiation along host plant lines has been clearly dem onstrated, and some gene flow betw een the forms is likely. However, the level of this gene flow has not been directly estimated, and there is evidence in that events after m igration (i.e. reduced survival of E. solidaginis hybrids) are likely to inhibit backcrossing.

How common are host races?

Phytophagous host races been confirmed in two of the 17 studies described here,

approxim ately 12% of the sympatric, host associated populations in w hich the possibility has been investigated. This conservative estimate is likely to rise by up to 40%, because current data suggests, or strongly suggests, the presence of seven more, w ithin aphids

Aphis citricola, Acythrosiphon pisum, and Cryptomyzus galeopsidis, the sawfly Platycampus luridiventris, the goldenrod gall ballmaker Eurosta solidaginis, and the apple maggot fly R.

pomonella vs. the flowering dogwood fly.

The studies carried out to date have concentrated on systems in w hich the presence of host races seems particularly likely, in most cases because there is evidence of host- associated differentiation between populations of a presum ed sympatric species. H ow common is cryptic, host-associated differentiation in phytophagous insects? Recent studies, benefiting from sensitive protein and DNA markers, suggest that this type of cryptic differentiation could be quite frequent in phytophagous insects. Taxonomic revisions of several taxa, by taking account of new behavioural and DNA or protein based characters, have detected sympatric, m onophagous biotypes w ithin presum ed polyphagous species - recent examples include bark beetles Dendroctonus brevicomis

(Kelley et al., 1999) and fruit flies of the genus Blepharoneura (Condon and Steck, 1997). Intensive studies of agricultural pests have also begun to uncover population

substructuring along host plant lines (e.g. Shufran et al., 2000), the starting point of m any studies described here. This differentiation does not seem limited to particular

phytophage taxa; a w ide variety of phytophage genera are represented.

Consequently, although the number of phytophage host races discovered so far is small, the num ber of insect systems that conceals them is potentially huge, and, if the pattern observed in the data presented here is representative of these systems, host races could indeed be a common phenomenon.

Because the shortage of detailed studies of host races noted earlier (Tauber and Tauber, 1989) continues, it is not possible to draw a more definite conclusion from the current empirical data. Of the 17 cases described here, there is insufficient data to resolve the status of about 10. The lack of data is most pronounced in relation to criteria (3.a) and (3.b), those dealing w ith the possibility of gene flow betw een the forms. M embers of m ost likely systems show differentiation along host plant lines, will interbreed in the

laboratory, and have levels of genetic differentiation compatible w ith continuing gene flow, b u t w hile several components of gene flow have often been studied, direct or indirect estimates of actual gene flow have rarely been obtained.

Is the formation of host races likely to lead to sympatric spéciation?

Did host races in sym patry today undergo their divergence during a period of isolation in allopatry? In the case of Rhagoletis pomonella on apple and haw thorn, at least, they did not. Historical records show that the apple host w as introduced w ithin the range of haw thorn, and it is extremely unlikely that the tw o were ever isolated in allopatry (Bush, 1969a; Bush, 1994).

A second line of evidence suggesting that host races are likely to form, and to diverge to the point of spéciation, is their geographical distribution. A vast num ber of host

specialised insect species have evolved - many millions of beetle species alone (Farrell, 1998). M any of these phytophagous insect species are identifiable prim arily by their host, and are capable of producing viable hybrids in the laboratory. Given their num bers, and the fact that sympatric spéciation in these groups is theoretically possible, a sympatric origin for these species seems much more parsim onious than the alternative, that all were separated and diverged in allopatry, obtaining their present sympatric distribution only as a result of secondary contact (Tauber and Tauber, 1989).

Further support for host shift spéciation has been provided by com parisons of several host associated biotypes in Rhagoletis. The observation that Rhagoletis pomonella and

Rhagoletis mendax, the apple and blueberry maggot flies, are good sym patric species, w hile host races of Rhagoletis pomonella exist on apple and haw , provides a continuity argum ent that the formation of host races in this genus is likely to lead to spéciation (Feder et al., 1998b; Payne and Berlocher, 1995). In addition, com parison betw een the

Rhagoletis suavis and pomonella species groups shows that members of the first have an allopatric or parapatric distribution, and are not specialized on different hosts, w hereas members of the latter, w hich are all sympatric, are also all restricted to different hosts (Bush and Smith, 1998). Again, this argues for an im portant role of host shift in the formation of sympatric Rhagoletis species.

Finally, the m ain difficulty for the theory of sympatric spéciation has always been to explain how selection can cause multilocus differentiation that is correlated w ith habitat

use and mate choice, in the presence of gene flow. By providing a continuum of examples in w hich host-associated differentiation is maintained in spite of actual or probable gene flow, the studies discussed here provide empirical evidence for a route to sympatric spéciation.