Introduction

Hybridisation in butterflies is not unusual (DESCIMON & MALLET, 2009). Hybrids between the two pierids Pontia daplidice (Linnaeus, 1758) and P. edusa (Fabricius, 1777) across a broad band in Italy have been known for some time (GEIGER et al., 1988; PORTER et al., 1997) and natural hybridisation events between species of Nymphalidae sensu lato have been reported relatively frequently. Recently, hybridisation between Melitaea phoebe ([Dennis & Schiffermüller], 1775) and M. ornata Christoph, 1893, in Hungary (VARGA, 1967; BÁLINT & ILONCZAI, 2001) and in Slovenia (RUSSELL et al., 2014) has been reported, although TÓTH et al. (2017: 276-277) suggested that hybridisation between these latter two species was unproven due to a lack of statistical analysis. Natural hybrids between Brenthis daphne ([Denis & Schiffermüller], 1775) and B. ino (Rottemburg, 1775) have also been recorded (KITAHARA, 2008).

In the Satyrinae, such events appear almost commonplace. They include Melanargia lachesis (Hübner, 1790) and M. russiae (Esper, 1783) (TAVOILLOT, 1967), Maniola telmessia (Zeller, 1847) and the endemic Turkish (geographically) M. halicarnassus Thomson, 1990 (THOMSON, 1990) and the Sardinian endemic Maniola nurag Ghiliani, 1852 and the widespread M. jurtina (Linnaeus, 1758) (GRILL et al., 2007). In view of the rather frequent observations of interspecific coupling reported (RUSSELL, 2013a, 2013b), it is surprising that more hybrids involving a M. jurtina parent have not been recorded.

In the Lycaenidae, hybridisation in polyommatine species has also been observed: between Lysandra bellargus (Rottemburg, 1775) and L. albicans (Gerhard, 1851) (GIL-T., 2007); L. bellargus and L. hispana (Herrich-Schäffer, [1851]) (CAMERON-CURRY et al., 1987). So far as the authors are aware hybridisation between Polyommatus icarus (Rottemburg, 1775) and P. celina (Austaut, 1879) has not been reported previously; this is perhaps unsurprising as they were only recognised as distinct species fairly recently. DINCĂ et al. (2011: 3931) suggested that genitalic differences between the two species were weak enough to present the possibility of hybridisation in the contact zone in southeastern Spain.

Materials, methods and observations

Two fresh male P. icarus (see Figs 1-5), originating from Devil’s Dyke, West Sussex. U.K., supplied by John Martin (Brighton, UK), and a single female P. celina (see Figs 6-9), reared by J. P. from stock originating from Playa Blanca, Lanzarote, Canary Islands, supplied by Martin GascoignePees (Stonesfield, UK) were released into a netted flower pot containing bird’s foot trefoil (Lotus corniculatus L., Fabaceae), a known host-plant of P. icarus in the UK (TOLMAN 2008: 156); in Lanzarote P. celina utilises a similar Lotus species, L. lancerottensis Webb & Berthel (TOLMAN, 2008: 156) but this was not available. The pot was placed in the sunshine and after a few days a large number of ova were deposited; however, viability proved to be very low. To avoid potential cannibalism, larvae were separated on emergence into small individual plastic boxes with a leaf of crown vetch (Securigera varia (L.) Lassen, previously known as Coronilla varia L.). The first author has reared both P. icarus and P. celina successfully on this plant, which has the advantage that, unlike Lotus corniculatus L., it does not produce toxins lethal to larvae if it is eaten extensively (pers. obs., first author). Twenty larvae were reared through to pupation and adults emerged successfully from all. A representative three pairs of hybrid butterflies were retained and are figured (see Figs 10-15).

Figs 1-8.–
1.P. icarus male upperside, Devil’s Dyke, West Sussex, U. K. 2.P. icarus male underside, Devil’s Dyke, West Sussex, U. K. 3.P. icarus female upperside, Devil’s Dyke, West Sussex, U. K. 4.P. icarus female underside, Devil’s Dyke, West Sussex, U. K. 5.P. icarus extended blue female, upperside, Devil’s Dyke, West Sussex, U. K. 6. P. celina male upperside, ex stock Playa Blanca, Lanzarote, Canary Islands. 7.P. celina male underside, ex stock Playa Blanca, Lanzarote, Canary Islands. 8.P. celina female upperside, ex stock Playa Blanca, Lanzarote, Canary Islands.

Figs 9-15.–
9.P. celina female underside, ex stock Playa Blanca, Lanzarote, Canary Islands. 10.P. icarus ♂ x P. celina ♀ F1 hybrid male upperside. 11.P. icarus ♂ x P. celina ♀ F1 hybrid male underside. 12.P. icarus ♂ x P. celina ♀ F1 hybrid female upperside. 13.P. icarus ♂ x P. celina ♀ F1 hybrid female underside. 14.P. icarus ♂ x P. celina ♀ F1 hybrid female upperside. 15.P. icarus ♂ x P. celina ♀ F1 hybrid female underside.

The remaining individuals were placed in a netted pot of bird’s foot trefoil and a very large number of ova resulted, almost covering the plant. In this case viability was extremely poor and only six larvae of this F2 generation hatched (from an estimated 1,000+ eggs). The larvae were placed individually into plastic pots each containing a leaf of crown vetch. However, none survived beyond the 1st instar. The plant used for ovipositing was searched several times over a period to see if any larvae had survived on the plant from unobserved ova; none were found. Thus the F1 hybrids were effectively infertile, demonstrating a post-copulative barrier.

Comments on hybrids between Melitaea phoebe and M. ornata

Offspring produced from a wild caught female M. ornata (RUSSELL et al., 2014: 137, fig. 2) from a population northwest of Rakitovec, Koper, Slovenia were considered by RUSSELL et al. (2014) to be naturally occurring hybrids with M. phoebe. TÓTH et al. (2017: 276-277) considered this had not been proven and that statistical analysis was required to confirm this was the case. We believe this to be unnecessary: the morphology of the larvae and adult butterflies place a hybrid source beyond doubt.

Larval survival was poor, the few surviving final instar larvae resulting from an egg batch of an estimated 60 ova produced by the female M. ornata had black head carapaces, suggestive of M. phoebe (Fig. 16); those of L4 + M. ornata larvae have brick red carapaces (Fig. 17). Two other females from the same M. ornata population also produced egg batches, from which the final instar larvae had the predicted brick red heads and from which the resultant butterflies had all the characteristics of M. ornata (RUSSELL et al., 2014: 137, figs 3-4). Underside hindwing characters of the hybrid adults displayed a mixture of characters between those of typical M. phoebe and typical M. ornata; antennae varied between the usual club shaped typical of M. phoebe (Fig. 18) and spatulate typical of M. ornata (Fig. 19). Only a single larva entered diapause; it began feeding the following spring and a vigorous female emerged (see Fig. 20), with wing and antennal morphology intermediate between its parents (RUSSELL et al., 2014: 140, fig. 9).

Figs 16-20.–
16.M. phoebe larva on Centaurea scabiosa Istria, Croatia, 27 July 2010. 17.M. ornata larva on Carduus collinus 2 km NW. of Rakitovec, Koper, Slovenia 30 April 2012. RV. 18.M. phoebe male underside, 2 km S. of Rakitovec, Koper, Slovenia (captured 17 May 2011). 19.M. ornata female underside, circa 2 km NW. of Rakitovec, Koper, Slovenia (captured 17 May 2011). 20.M. phoebe ♂ x M. ornata ♀ female hybrid underside, reared ex M. ornata female from Rakitovec, Slovenia, emerged 17 May 2012.

It was noted (RUSSELL et al., 2014: 137) that there were at least three populations of M. phoebe within a few kilometres of and surrounding the studied M. ornata population: 2 km south of Rakitovec, 3.5 km north of Rakitovec and just south of Podpec; the last site being less than 2km distant from the M. ornata population under study. It is noteworthy that the flight time of M. phoebe is approximately two weeks later than M. ornata and males of M. phoebe were captured from these three locations at the same time that freshly emerged M. ornata females were present at the study site. Thus, we believe that M. phoebe males from any of these surrounding populations are quite likely to have encountered a female M. ornata whilst searching for a mate. The authors are confident that hybrids between M. phoebe and M. ornata were the result of a female M. ornata impregnated by a M. phoebe male at this Slovenian locality.

Discussion and conclusión

PART 1. POLYOMMATUS

Distribution of P. celina includes the Canary Islands, North Africa (Morocco, Algeria, Tunisia and Libya(?)), southern Portugal, southern Spain (including the Balearic Islands), Malta, Sardinia and Sicily; the widespread P. icarus is present in both Spain and Portugal but not in other areas where P. celina flies. Thus there is potential for natural hybridisation only on the Iberian Peninsula; particularly in an area near Madrid where the species are sympatric (CARRILLO et al., 2017). The results of this experiment suggest that F1 hybrids are quite likely to occur naturally in the zone of sympatry, where emergences of the two species are at least partially synchronic. Confirmation of such a hybridisation event is unlikely from casual field observation due to the fact that separation of the two species, let alone hybrids, is virtually impossible in the field. Separation can only be made with certainty by genitalic dissection and/or molecular analysis (DINCĂ et al., 2011).

PART 2. MELITAEA

We consider it possible that the close proximity of these two species, coupled with the fact that M. phoebe males must often emerge at a time when the only females available are those of M. ornata, may regularly present the opportunity for natural hybridisation. Further, we consider that natural hybridisation between a male M. ornata and a female M. phoebe is unlikely in the populations studied because when female M. phoebe emerge most male M. ornata will have already mated and died. However, if both species were reared in captivity and M. ornata males introduced to a female M. phoebe, then hybridisation is conceivable, even probable. Considering that natural hybrids between closely related species have been recorded so frequently, it is a mystery why the unmistakably hybrid offspring obtained from a female M. ornata taken from the colony near Rakitovec should be questioned (TÓTH et al., 2017: 276-277). Particulary when those authors agreed that the two species hybridised previously (TÓTH et al., 2017: 277), resulting in shared COI haplotypes between western populations of M. ornata and M. phoebe occitanica Staudinger, 1871 [Type Locality: Barcelona; cf. VERITY (1928: 163), VAN OORSCHOT & COUTSIS (2014: 60) and RUSSELL et al., 2020: 500-501 and Figs 5-7]. The distribution of M. phoebe occitanica, distinguishable from M. phoebe phoebe by its later instar larvae (RUSSELL & TENNENT, 2016: 43) and in adults using electrophoresis (PELTZ, 1995) is established.

Historical distribution and identification is somewhat confused since M. ornata was recently “discovered” in Spain (SÁNCHEZ-MESA & MUÑOZ-SARIOT, 2017), although it occurred there previously at least from the early part of last century. Specimens taken by Romei in 1925 in the Sierra Nevada and given subspecific status, as M. phoebe bethunebakeri by De Sagarra in 1926, were in fact M. ornata (see RUSSELL et al., 2020: 196 and figs 14a, b and c). The proximity of some populations of M. ornata to those of M. phoebe in Spain certainly provides the potential for hybridisation.

Acknowledgments

Thanks are due to John Martin (Brighton, U. K.) for providing fresh males of P. icarus from Devil’s Dyke near Brighton, Sussex, U. K. and to Martin Gascoigne-Pees (Stonesfield, U. K.) for providing the first author with females of P. celina from stock originating from Playa Blanca, Lanzarote (Canary Islands, Spain). The authors would also like to thank Enrique Garcia-Barros (Madrid, E.) for his useful comments on the manuscript, which led to improvements to the article. Rudi Verovnik (Ljubljana, Sl) is thanked for the photograph of the M. ornata larva.

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Notas de autor

Autor para la correspondencia / Corresponding authorpeterjcrussell@yahoo.co.uk