R. Cianchi, A. Ungaro, M. Marini, and L. Bullini. Molecular Ecology, 12 (6):
1461-1471(2003)In swallowtails, uses "nine fully diagnostic and two differentiated allozyme loci and a mitochondrial DNA marker" to find that "Levels of introgression were low (0–1%) at two sex-linked loci and mitochondrial DNA, limited (0.4–2%) at three autosomal loci coding for dimeric enzymes, and high (up to 43%) at four autosomal loci coding for monomeric enzymes."
Hybrids were identified by morphology. Deduction of introgression was based on seeing mixed types in one species and not the other..
S. Vollmer, and S. Palumbi. Science, 296 (5575):
2023-2025(2002)Shows with nuclear and mitochondrial markers that a "species" is actually a F1 hybrid that doesn't contribute much to the gene pool but may live potentially forever as asexually reproducing colonies. Shows unidirectional mtDNA introgression on the basis of one species having different mtDNA types and the other only one..