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BIOSYSTEMATICS OF THE GENUS EUXOA (LEPIDOPTERA: NOCTUIDAE). XVIH. COMPARATIVE BIOLOGY AND EXPERIMENTAL TAXONOMY OF THE SIBLING SPECIES EUXOA RIDMGSIANA (GRT.) AND EUXOA MAIMES (SM.)

Published online by Cambridge University Press:  31 May 2012

J.R. Byers ,
D.L. Struble  and
J.D. Lafontaine
J.R. Byers
Affiliation:
Agriculture Canada Research Station, Lethbridge, Alberta, CanadaT1J 4B1
D.L. Struble
Affiliation:
Agriculture Canada Research Station, Lethbridge, Alberta, CanadaT1J 4B1
J.D. Lafontaine
Affiliation:
Biosystematics Research Institute, Agriculture Canada, Ottawa, Ontario, CanadaK1A 0C6
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Abstract

The species previously recognized as Euxoa ridingsiana (Grt.) is shown to be composed of a sympatric pair of sibling species, Euxoa ridingsiana (Grt.) and Euxoa maimes (Sm.), which in the laboratory will produce viable F1 hybrids but no F2. Results of F1 sib and backcrosses show that the F1 males are fertile and the F1 females are infertile. In mating-bias tests conducted in laboratory cages, 74% of matings were conspecific and 26% interspecific. Differences in the diel periodicities of mating, which are about 2 h out of phase, may account for the mating bias. The duration of development of E. ridingsiana in the laboratory and its seasonal flight period in the field are about 2 weeks in advance of that of E. maimes. However, there is considerable overlap of the flight periods and, with the tendency of females of both species to mate several times, it is unlikely that the difference in seasonal emergence is enough to effect reproductive isolation. It is evident that, under natural conditions, reproductive isolation can be maintained entirely by species-specific sex pheromones. This mechanism of reproductive isolation is, however, apparently ineffective when moths are confined in cages in the laboratory.

Biogeographic considerations suggest that the differences in life-cycle timing and mating periodicities might have been adaptations to adjust development and reproduction to prevailing ancestral environments. If the initial differentiation of the 2 species occurred in isolation and included at least an incipient shift in the pheromonal mate-recognition system, it is possible that upon reestablishment of contact between ancestral populations the differences in life-cycle timing and mating periodicities acting in concert could have effected substantial, albeit incomplete, reproductive isolation. Subsequent selection to reinforce assortative mating to preserve coadapted gene complexes could then have resulted in differentiation of discrete pheromonal systems and attainment of species status.

Résumé

On démontre que l'espèce auparavant reconnue comme Euxoa ridingsiana (Grt.) est en fait composée de 2 espèces jumelles, Euxoa ridingsiana (Grt.) et Euxoa maimes (Sm.), qui produisent des hybrides viables à la F1 mais pas à la F2. Les résultats du croisement des progénitures de la F1 et de rétrocroisements montrent que les mâles de la Fl sont fertiles et que les femelles de la F1 sont infertiles. Des testes de symétrie de l'accouplement dans des cages au laboratoire ont révélé 74% de croisements conspé-cifiques et 26% de croisements interspécifiques. Une différence dans la périodicité de l'accouplement, un décalage d'environ 2 h, est possiblement la cause du biais observé. La durée du développement de E. ridingsiana au laboratoire et sa période saisonnière de vol sur le terrain sont environ 2 semaines en avance sur celles de E. maimes. Cependant, il y a un chevauchement considérable des périodes de vol, et à cause de la tendance des femelles des 2 espèces à s'accoupler plusieurs fois, il semble que la différence dans leur émergence saisonnière n'est pas suffisante pour assurer l'isolement reproducteur. Il est clair qu'en conditions naturelles, l'isolement reproducteur peut être réalisé par des phéromones sexuelles spécifiques des espèces. Ce mécanisme serait appa-ramment inefficace lorsque les papillons sont gardés en cages au laboratoire.Certaines considérations biogéographiques suggèrent que les différences au niveau de la chronologie du cycle vital et de la périodicité de l'accouplement refléteraient desadaptations du développement et de la reproduction à des milieux ancestraux. Si la différenciation initiale des 2 espèces s'est produite isolément, après au moins un début de séparation de leurs phéromones de rapprochement des sexes, il est possible que lors du rétablissement du contact des populations ancestrales, les différences existantes aux niveaux de la chronologie du cycle vital et de la périodicité de l'accouplement aient conduit à un isolement reproducteur prononcé bien qu'incomplet. Une sélection subséquente pour l'assortiment de l'accouplement de façon à préserver les complexes de gènes coadaptés, a pu résulter par la suite en la différenciation de systèmes discrets de phéromones et l'atteinte du statut d'espèces distinctes.

Type
Articles
Information
The Canadian Entomologist , Volume 117 , Issue 4 , April 1985 , pp. 481 - 493
Copyright
Copyright © Entomological Society of Canada 1985

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