Interspecific hybridisations in natural populations of ants by example of a regional fauna (Hymenoptera, Formicidae)

Abstract

The identification of hybrids by high-precision stereomicroscopy and chorological data is described in detail by example of the ant species Formica bruni and pressilabris. In a synopsis of heterogenous data, the overall hybrid frequency in the ant fauna of Central Europe is estimated. 17 of the 164 ant species of this region are demonstrated and further 2 species strongly suspected to hybridise. The lowfrequency hybridisers, showing overall hybrid ratios < 3%, were native elements of the Central European fauna before the onset of human cultivation and experienced direct interspecific contact for longer periods of their natural history. They developed more effective mechanisms of reproductive isolation beginning at the prezygotic level. Extremely high (12-31%) local hybridisation ratios occurred in species that invaded the area after anthropogenic changes in landscape structure. The segregated distribution of invaders and autochthonous species in the precultural period apparently did not impose the need to evolve more effective mechanisms for reproductive isolation. Prezygotic mechanisms in particular are deficient. In local situations, 19% of Lasius jensi matings and 44% of Leptothorax albipennis matings leading to successful nest foundations were matings with heterospecific partners. Signs for a dissolution of interspecific phenotypic differences are not detectable in most of the species. The dispersal of hybrid genotypes is apparently inhibited. Factors that probably stabilise the genomic integrity of hybridizing parent species are: (a) inability to produce hybrid queens (in L. jensi ¥ umbratus), (b) aneuploidy of F 1 females with inability to perform a balanced meiosis (in hybrids of L. albipennis with 3 other species), and (c) selection against hybrids in the epigenetic environment of alleles (Leptothorax nylanderi ¥ slavonicus). Extreme ratios of heterospecific matings would mean a dangerous drain of genetic material. A mechanism to reduce these losses is postulated, consisting of a 'cleptogamy' (theft of heterospecific sperm) by queens that missed a conspecific male, an establishment of a functioning colony of F 1 hybrid workers, a depression of the rearing or fertility of hybrid queens, and a maximum production of own sons by the queen. This mechanism could explain the persistence of rare species (Lasius jensi or Leptothorax albipennis) in an environment of more populous heterospecific hybridisation partners.

Low genetic diversity in Polish populations of sibling ant species: Lasius niger (L.) and Lasius platythorax Seifert (Hymenoptera, Formicidae)

“…The recent morphological and genetic data revealed the surprising commonness of hybridization in ants (Feldhaar et al, 2008;Seifert, 2009), also among members of the genus Lasius (Pearson, 1983;Seifert, 1999;Umphrey, 2006). It seems plausible that in case of recently diverged species, as L. niger and L. platythorax, the reproductive barriers cannot completely prevent the possibility of interspecific hybridization.…”

“…It seems plausible that in case of recently diverged species, as L. niger and L. platythorax, the reproductive barriers cannot completely prevent the possibility of interspecific hybridization. There are reports that both species L. niger and L. platythorax may hybridize with other ant species, i.e., L. niger with L. alienus (Pearson, 1983) or L. psammophilus (Seifert, 1999), and L. platythorax with L. emarginatus (Seifert, 1999). There are even some observations of morphologically intermediate forms between L. niger and L. platythorax inhabiting the moss-covered rock cracks in Finland (W. Czechowski, pers.…”

“…In case of L. niger and L. platythorax, both species occur in close geographic proximity and also our observations confirm their sympatry; the colonies of both species can be situated even less than 50 m apart. On the other side, ant species which maintained interspecific contact during their natural history have built up more effective mechanisms of prezygotic isolation than species that rarely or never experienced such contact (Seifert, 1999). In consequence, the observed frequency of hybridization is lower in the first group of species (Seifert, 1999).…”

“…On the other side, ant species which maintained interspecific contact during their natural history have built up more effective mechanisms of prezygotic isolation than species that rarely or never experienced such contact (Seifert, 1999). In consequence, the observed frequency of hybridization is lower in the first group of species (Seifert, 1999). The chances for interspecific hybridization may also depend on the time limitation for mating activities.…”

Lifelong commitment to the wrong partner: hybridization in ants

“…Hung & Vinson 1977;Ross et al 1987;Vander Meer & Lofgren 1989;Helms Cahan & Vinson 2003;Shoemaker et al 2006) and several other ant genera (Lasius : Pearson 1983b, Seifert 1999, Umphrey 2006Formica: Seifert 1999, Seifert & Goropashnaya 2004Temnothorax: Douwes & Stille 1991, Seifert 1999, Pusch et al 2006aPogonomyrmex: Helms Cahan et al 2002, Julian et al 2002, Schwander et al 2007bCrematogaster: Feldhaar et al 2003, H. Feldhaar 2006. It appears that in some areas of Central Europe, almost half of all female sexuals of the well-studied and highly diverse ant genus Temnothorax mate with allospecific males (Douwes & Stille 1991;Seifert 1999). Morphological studies and the narrowness of hybrid zones suggest that most cases of natural hybridization do not result in either the breakdown of species borders or the evolution of highly viable, permanent hybrid species with the exception of some lineages of Pogonomyrmex harvester ants (see below).…”