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Sympatric speciation, in which new species arise without geographical isolation (Doebeli, M. (1999), tends to be highly debatable for many biologists when comparing it to Allopatric Speciation, the divergence of species subsequent from geographical isolation. Speciation must demonstrate reproductive isolation, species sympatry, sister relationships and that an earlier allopatric phase is very unlikely (Lexer, C. (2006). A stable polymorphism can occur in a heterogeneous environment separated into two niches between two alleles each conferring a selective advantage in one of the niches, even if adults from the mating population provided the population size to be separately regulated in the two niches and the selective advantage is large (Smith, J. (1966). When females lay their eggs in the niche in which they themselves were raised a stable polymorphism occurs. Four genetic mechanisms which can cause mating isolation are as follows: pleiotropic genes, modifying genes, habitat selection and assortative mating genes are considered. Therefore, stable polymorphism could be the first stage in sympatric speciation. Recent research on natural host races and sympatric sister species, theoretical models, lab experiments and comparative phylogenetic analysis has greatly strengthened the event of sympatric speciation (Gies, T. (2018).
A major cause of biodiversity is biodiversity itself (Feder, J. (2018). As new species form, they can generate niches that others may exploit, which can catalyse a sequence reaction of speciation events across trophic levels (smith, J. (2018). They conducted experiments and tested for sequential radiation in the apple maggot fly, Rhagoletis pomonella complex, an insect which confers sympatric speciation in the absence of geographic isolation due to host plant shifting. This species is attacked by three species of braconid wasps such as Diachasma alloeum, Utetes canaliculatus and Diachasmimorpha mellea (Wharton and Marsh, (1978).
There are two questions that must be asked in this case study, does the wasp species D. alloeum form ecologically and genetically differentiated incipient species in response to their diversifying Rhagoletis fly hosts and does the same host-plant-related adaptations that serve as ecologically-based gene flow barriers between flies have also evolved to reproductively isolate the wasps (FORBES, A. (2010). As a result of specialising on variable fly hosts, including the apple infesting race R. pomonella shows that the parasitic wasp Diachasama alloem formed new developing species. Furthermore, the case study displayed traits that differentially adapt R. pomonella flies to their host plants have rapidly evolved and therefore are served as biological barriers for reproduction which in turn isolates the wasps (Smith, J (2018). In conclusion, speciation cascades as the effects of new niche construction move across trophic levels (Forbes, A. (2018).
D. alloeum is in the genus Diachasma which also belongs to the ferrugineum species as well as D. ferrugineum and D. muliebre which are widespread in North America (Wharton, 1997). Surveys of D.alloeum (Stelinski et al, 2004) shows that the Diachasma species can attack a subsection of R. pomonella sibling species complex. Furthermore, R. pomonella, both hawthorn and apple fly races, is parasitized by D. alloeum, R. mendax (blueberry maggot) and R. zephyria (snowberry maggot) are also parasitized by D. alloeum. However, D. alloeum does not take place on R. pomonella’s entire environmental range, the wasp is situated in the north-eastern and Midwestern portion of R.pomonella’s distribution in the U.S.A (Rull et al., 2009). The parasitic wasp D. alloeum was tested for sequential radiation by examining if wasps attacking the derived R. pomonella and the ancestral hawthorn, and their closely related sibling species which are R. mendax and R.zephria show forms of host related genetic variation (Feder, J. (2018).
They also investigated if wasps varied from the same host plant, which showed genetic differentiation in the D. alloeum population, which appeared to be similar in the fly species R. pomonella. Mitochondrial DNA (mtDNA) cytochrome oxidase I (COI) sequences displayed only modest host related differentiation of wasps (Smith, J. 2018). Which concludes that apple, hawthorn, blueberry and snowberry wasps are of recent origin which in turn do not have highly genetically diverged cryptic sibling species (Forbes, A., Powell (2018). Instead, these taxa of recent origin have different haplotype frequencies. D. ferrugineum, D. alloeum and D. muliebre are distinguished by ∼5% mtDNA divergence (Feder, J. 2010). We can argue that in snowberry wasps, they found mtDNA haplotype that is not present in any of the other wasp populations. In addition, the mtDNA haplotype found in blueberry, apple and hawthorn wasps was not present in the snowberry wasp, which tells us that the snowberry wasp was offset from the other taxa (Feder, J. (2018). Apple, blueberry, hawthorn and snowberry fly populations of D.alloeum show genetic differentiation for 9 of 21 microsatellite loci was scored across sympatric sites in the U.S.A (Forbes et al., 2009). Neighbour-joining trees for the microsatellites separated blueberry and hawthorn wasp populations at different ends of the networks (Feder, J. (2010). Blueberry wasps were most closely related with snowberry wasps, whilst the apple wasps were in between hawthorn and blueberry populations.
Similar host-related adaptations that naturally isolate R. pomonella flies seems to play a huge role in genetically differentiating D. alloeum wasps. Field studies were formed of D. alloeum which showed that adult wasps use host fruits as a site of mating which is similar to the flies. They also displayed similar discriminatory behaviour for host fruit volatiles to Rhagoletis flies in Y-tube olfactometer assays (Forbes et al., 2010). In the case study they found that the hawthorn, apple, snowberry and blueberry wasp populations all positively slanted towards the arm of the Y-tube which contained their natal fruit odour and were also antagonised by non-natal volatiles (forbes et al., 2010). The finding of avoidance behaviour in D. alloeum is very interesting as R. pomonella flies are also deterred by non-natal volatiles (Forbes et al., 2005). Because the F1Rhagoletis hybrids between hawthorn and apple flies fail to respond to any fruit volatiles (Linn et all., 2004). They have hypothesised that the evolution of avoidance to alternate hosts may cause olfactory incompatibilities and constitute a previously unrecognised post zygotic barrier in flies of mixed ancestry (hybrids are partly ‘behaviourally sterile’ due to a reduced chemosensory ability to find host fruit for mating and laying eggs (Feder & Forbes, 2008).
Diapause life‐history differences represent a second ecological barrier to gene flow among D. alloeum wasps, which is the case for the flies (Forbes et al., 2009). Sympatric apple, blueberry and hawthorn flies all hatch as adults at different times in the summer and/or spring, this is because they match the time of fruit ripeness on their host plants (Smith, J. (2018). Field studies, lab rearing and many experiments showed that the parasitic wasp of D. alloeum have a similar hatching time differences among the apple, blueberry and hawthorn wasps. Another study showed that the estimated mean longevity of adult wasps is less than half of that of flies, in other words it takes wasps two weeks and flies up to a month. Thus, temporal isolation between wasps can be more pronounced than for flies (up to 75%; Forbes et al., 2009). They tested for the microsatellite loci separately in the female and male wasps because there were slight differences in the hatching time between the two, male wasps were hatching several days prior to the female wasps (Smith, J. (2018). For D. alloeum the microsatellite loci were dramatically correlated with variation in the hatching time for the blueberry and hawthorn wasps. The hatching time was moderate in female apple wasps and was not a significant analyst for hawthorn.
For R. pomonella, six allozyme loci which displaced host-related frequency differences all correlate with hatching time (feder et al., 1993; 1995), which in turn ties together the genetics of host race formation with traits that isolates the flies. They discovered that comparable relationships exist in D. alloeum, with the microsatellite expanding <50% of the variation in hatching time in wasp populations (forbes et al, 2009). In particular, the allele 196 at locus 3 was strongly related with earlier hatching in apple and blueberry wasps. Locus 3 therefore represents a naturally segregating, major affect QTL for diapause associated with ecological reproductive isolation among D. alloeum populations (Forbes, A. (2010).
In this study, they documented the alleles 196 + 200 + 204 at locus DA003 related to earlier hatching times in both male and female blueberry and apple wasps (Stelinski, L. (2018). With the allele 196 common in apple, blueberry and snowberry wasp populations, but absent in all hawthorn wasps. These three alleles were formed from the highest frequency (n=205) from the blueberry wasp, intermediate for the apple wasp (n=260) and the lowest frequency for the hawthorn (n=385) which shows the order of emerging time from earliest to the latest for this population (Stelinski, L. (2018). Little or no gene flow occurs from the blueberry or apple wasps because of the lack of allele 196 in hawthorn wasps (R. mendax (2010).
In conclusion, the results of the articles show that sympatric host shifts of R. pomonella onto the new plants initiated a rapid burst of adaptive radiation for its parasite, D. alloeum. They also discovered that host related environmental effects which introduced speciation for Rhagoletis moved through the community and may have enlarged the diversity of the wasp. Lastly, the lack of the 196 allele at microsatellite locus DA003 in the hawthorn wasp shows it is not the single ancestor of the apple wasps which therefore implies a blueberry wasp derivation.
Doebeli, M. (1999). On the origin of species by sympatric speciation. International journal of science, 51(6).
Forbes, A., Powell, T., Stelinski, L., Smith, J. and Feder, J. (2018). Sequential Sympatric Speciation Across Trophic Levels.
FEDER, J. and FORBES, A. (2010). Sequential speciation and the diversity of parasitic insects. Ecological Entomology, 35, pp.67-76.
Gies, T. (2018). The ScienceDirect accessibility journey: A case study. Learned Publishing, 31(1), pp.69-76.
Lexer, C. (2006). Sympatric speciation in palms on an oceanic island. International journal of science, 119(3100).
Smith, J. (1966). Sympatric Speciation. The American Naturalist, 100(916), pp.637-650.
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