DEVELOPMENTAL STABILITY, HETEROZYGOSITY, AND FITNESS IN 2 INTRODUCED FIRE ANTS (SOLENOPSIS-INVICTA AND SOLENOPSIS-RICHTERI) AND THEIR HYBRID

Fluctuating asymmetry of seven bilateral morphometric characters was studied in two introduced fire ant species, Solenopsis invicta and S. richteri, and their hybrid as a means of assessing relative levels of developmental stability. All study populations exhibited significant fluctuating asymmetry for at least three of the seven characters. Among these populations, a hybrid population from Mississippi, U.S.A. exhibited generally high levels of fluctuating asymmetry relative to the parental populations, including a highly significant elevation in asymmetry for one leg character. The S. invicta study population generally displayed the lowest levels of fluctuating asymmetry over all characters, as well as the lowest degree of anomalous wing venation. Genomic incompatibility between the parental fire ant species apparently overrides any heterotic effects arising in the regulation of development in the more highly heterozygous hybrids, and thus may create weak selection against hybrid genotypes. Nonetheless, this incompatibility is not of sufficient magnitude to constitute a significant postzygotic isolating mechanism. The dynamic nature and substantial width of the fire ant hybrid zone in the U.S.A may be interpreted as resulting from the interaction of (1) a dominant influx of S. invicta alleles from the south associated with superior fitness (greater developmental stability) of this form and (2) a barrier to gene flow between the species caused by weak selection acting against hybrid genotypes. © 1990 The Genetical Society of Great Britain.