Bad species : 4. Zerynthia rumina and Z. polyxena: relativity of mixiological criteria

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The genus Zerynthia contains two species, both recognized since the dawn of entomology, Z. rumina, a Western Mediterranean species, and Z. polyxena, Eastern Mediterranean. They overlap in Southern France, where they display marked ecological differentiation, while in areas where only one species is found, both have a more extensive niche. Besides wing pattern differences, there are diagnostic alleles between, with Nei’s D ≈ 0. 80, (Braconnot, unpublished) and strong divergence in
mitochondrial and nuclear gene sequences (Nazari et al., 2007) – there is no doubt they are “good” species! Both display marked intraspecific differentiation: wing patterns of the French subspecies Z. rumina medesicaste and Z. polyxena cassandra
clearly differ by wing pattern from their respective nominal subspecies, but variation forms a wide cline within a continuous distribution.
Natural hybrids between the species are scarce (only five are known to HD), but interspecific pairing has been observed in the field (de Puységur, 1947). A large series of crosses within and between species has been performed by HD, although only some have been published (Descimon & Michel, 1989). When Z. rumina medesicaste was crossed with Z. r. rumina, remarkable hybrid vigour was observed in the F1, followed by strong hybrid breakdown in the F2 (i.e. F1 x F1): arrested embryonic development, larval weakness, and difficulties of pupation. Less than 5% of ova reached the adult stage in about 10 parallel broods. The low viability persisted in further crosses; only backcrosses, with either parent subspecies (or, paradoxically, with Z. polyxena), restored viability. Crosses between Z. p. polyxena from Greece and Z. p. cassandra from southern France also produced F1 hybrid vigour, and some F2 (F1 x F1) hybrid breakdown. However, the F2 viability was not too low (around 25%), and further crosses (F2 x F2 and more) displayed markedly enhanced viability: incompatibility therefore seemed less marked than in the first case. Crosses between Austrian and French Z. polyxena produced no F2 hybrid breakdown. Mate choice was studied in cages containing 10 males and 10 females of each species. Only intraspecific matings were observed (including the aforementioned
distinct subspecies), demonstrating strong prezygotic barriers between species. All females proved to have mated, and one female polyxena produced offspring consisting partly of polyxena and partly of hybrids. Clearly, she had mated twice, and
with males of each species. The hybrids were viable, but while the F1 x F1 cross resulted in no offspring, backcrosses with polyxena and rumina were successful. The backcross hybrids from either side could, however, be crossed with the more distant
parental strains. Thus backcrossed individuals, which had 3/4 of their genes from one species and 1/4 from the other, gave symmetrical F3 progeny with 3/8 rumina - 5/8 polyxena offspring and the reciprocal; the same scheme was applied in F4 and
beyond. The possibilities for complex crosses increased with the rank of hybridization and some were practised (for a complete account, see Descimon & Michel, 1989). The hybrids were viable provided they had at least one complete unrecombined
genome from a parental strain. Much more surprisingly, two later hybrid x hybrid crosses (not many were tried) gave fairly viable offspring, with no significant departures from 1:1 sex ratio or diapause abnormalities. In spite of strong pre-mating
isolation between the pure species, female hybrids were attractive to males of either species, and male hybrids were attracted to any female. Similar results on hybrid sexual attractiveness have been obtained in a number of other butterfly species; e.g.
Heliconius (McMillan

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et al., 1997; Naisbit et al., 2001).
It was not possible to continue the crosses, but some clear facts emerge. First, F2 hybrid breakdown is not absolute in interspecific crosses; second, it is not limited to interspecific crosses; it may take place between subspecies, as is known in other
species (e.g. Oliver, 1972, 1978; Jiggins et al., 2001). The latter fact is particularly paradoxical, since, within both species, broad, clinal, unimodal hybrid zones connect “incompatible” populations. Careful field work could well disclose interesting features in these contacts. Hybrid inviability is therefore probably not a useful species criterion on its own in crosses between geographically distant taxa. The ease of playing ping-pong with the two species once initial barriers have been ruptured shows that there is no absolute threshold of post-zygotic incompatibility at the species level.

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