On aphids, their host plants and speciation : a biosystematic study of the genus Cryptomyzus
Allozyme data as determined by starch gel electrophoresis revealed that all species of Cryptomyzus could be distinguished on the basis of unique alleles. Moreover, differences were detected between the two forms of C.alboapicalis, which do not host-alternate and instead live on Lamium album and L. maculatum , respectively. In the C. galeopsidis complex as well a form that host-alternates between Ribes rubrum and L. galeobdolon was distinguished. The two non- alternating subspecies on R. rubrum and R.nigrum, respectively, differed in allozyme frequency, indicating a reduced gene flow. Electrophoresis proved a powerful tool in unraveling relationships of closely related aphid complexes (Chapter 1).The life cycle of C.heinzei was elucidated by the discovery that Ribes alpinum is the winter host and Stachys officinalis the summer host. No populations survived on the originally described summer host plant of this species, Satureja vulgaris , probably indicating that the host plant was initially misidentified. In laboratory experiments R. alpinum appeared to be the winter host of C.ballotae although reproduction and developmerit on this plant were weak. A full account of the host plants of Cryptomyzus species is given in Chapter 2.Closely related forms of C. alboapicalis and C.galeopsidis revealed a definite host plant preference for their own particular summer host plant. This fact was corroborated by the host plant suitability, based on the reproductive performance of these forms. Significantly, for the development of reproductive isolation those morphs returning to the winter host exhibit a prefererence for the host on which their stem mother was born. Experiments confirmed this feature in the case of host- alternating clones of C.galeopsidis from R. nigrum, although host preference of those from R. rubrum proved to be ambiguous. Several clones preferred R. rubrum on which their oviparae matured, while others preferred R.nigrum on which their oviparae matured as well. This indicates that gene flow occurs between these forms. The populations with intermediate behaviour may be assumed to be hybrids and the forms on R. rubrum and R. nigrum are considered to represent host races (Chapter 3).Hybridization experiments were performed between the closely related forms of C. galeopsidis, which share the winter host R. rubrum, but have different summer hosts, Galeopsis and Lamium galeobdolon , respectively. A F 1 generation could be established, but its fecundity on both summer hosts was lower than that of the parents. The F 2 and a backcross revealed that reproduction on the winter host was weak and the populations subsequently died out. This demonstrates that hybridisation of these two forms in the field would probably not result in permanent populations and natural selection would eliminate these hybrids.F 1 hybrids were also used to determine the genetic basis of reproductive performance and host preference. This objective could not be fully realized, because of hybrid inferiority. Preliminary results indicate that preference may be determined by only a few genes and reproductive performance by many.Hybrids between the host-alternating and non-alternating forms of C.galeopsidis revealed no hybrid inferiority, and probably hybridization also occurs in the field. The results of these crosses argue for a one gene (complex) determination of host-alternation. The implications of this for speciation are discussed (Chapter 4).A morphometric study was initiated to determine whether the forms of C.alboapicalis and C.galeopsidis, described on the basis of their hosts and life cycles, are also morphologically distinct. C.alboapicalis from L.album can be differentiated by the greater number of hairs on their abdominal segments. A canonical variate analysis using wingless females showed that C.alboapicalis on L. maculatum deviates considerably in morphology from the other taxa. A linear discriminant function, which uses the best four characters, adequately separates this taxon. C.galeopsidis on L. galeobdolon is more closely related to the other C.galeopsidis forms, and the linear discriminant function is less reliable. The four host-alternating and non-alternating forms of C.galeopsidis are closely related, and there is no unequivocal morphometric support for the separation of the forms on R. rubrum and R. nigrum (Chapter 5).The taxonomic conclusions that emerge from this study are:* C. (Ampullosiphon) stachydis (Heikinheimo) belongs to Cryptomyzus on the basis of its host plant relationships (Ribes and Labiatae) and the presence of a filterchamber in its gut.* C. heinzei Hille Ris Lambers is a separate species which differs from the morphologically comparable C.korschelti Börner in having different host plants and unique allozymes.* The form of C.alboapicalis that only lives on L. maculatum is a separate species with the name C.ulmeri Börner It was previously synonymized with C.alboapicalis (Theobald) that lives on L. album but it differs from this species and C.galeopsidis (Kaltenbach) forms in its host plant preference, reproductive performance, life cycle, allozymes and morphometrics.* The form of C. galeopsidis that host-alternates between R. rubrum and L. galeobdolon is a separate species and described as C.maudamantisp.n. . Its host plant preference, reproductive performance, allozymes and the inferiority of its hybrids conclusively show that it differs from C.galeopsidis sensu strictu.* Within C.galeopsidis the forms on R. rubrum can be distinguished from those of R. nigrum. On the basis of host preference, reproductive performance and allozyme frequency these forms are called host races. Contrastly, there is no support for taxonomically separating the host-alternating and non-alternating forms living on the same primary host plant. This makes the use of subspecific names for the non alternating forms, C.g. citrinus HRL on R. rubrum and C.g. dickeri HRL on R. nigrum , undesirable.* A key for wingless and winged virginoparous females was constructed for all European species of Cryptomyzus (Chapter 5).The possible pathways of speciation in Cryptomyzus are discussed. Therefore, a phylogeny was presented for this genus, based on allozyme, life cycle and morphological characters. A close association between the taxonomic relationships of Cryptomyzus and its host plants appeared, which was suggested to have been originated by sequential evolution (Jermy 1984). The process of speciation is discussed more generally, and the role of a host plant shift is considered. Because aphids have cyclical parthenogenesis and a close association with a specific host plant, they seem good candidates to follow the mode of sympatric speciation (Chapter 6).
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| Format: | Doctoral thesis biblioteca |
| Language: | English |
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Landbouwuniversiteit Wageningen
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| Subjects: | aphididae, aphidoidea, biological nomenclature, classification, evolution, herbivores, taxonomy, biologische naamgeving, classificatie, evolutie, herbivoren, taxonomie, |
| Online Access: | https://research.wur.nl/en/publications/on-aphids-their-host-plants-and-speciation-a-biosystematic-study- |
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