Gene flow across a chromosomal tension zone. I. Relicts of ancient hybridization

Abstract

Two parapatric subspecies of grasshopper with extensive karyotypic differences form a hybrid zone in which the change-over of chromosomal characters occurs over a distance of 800 m. Asymmetrical introgression of restrictionfragment markers of the nuclear ribosomal RNA genes, and mitochondrial DNA, and also of four enzyme electromorphs is reported. These markers were found to have introgressed for varying distances (100-300 km) to the north of the present-day hybrid zone. It is proposed that these markers are relicts of ancient hybridization between the Moreton and Torresian subspecies in an area where only the Torresian form (as defined by karyotype) is now found, and that the two taxa have maintained their chromosomal distinction despite prolonged hybridization and the geographic displacement of the Moreton subspecies by the Torresian form.

Empirical study of hybrid zone movement

“…Below, I describe three well-studied cases where zone movement has been postulated and discuss the difficulties involved in interpreting the evidence, then list 13 other possible cases. Details of these 16 hybrid zones can be found in Supplementary Table 1. A sharp hybrid zone north of Brisbane between 'Moreton' and 'Torresian' chromosomal races of the grasshopper Caledia captiva appears to be a tension zone, but 'Moreton' rDNA and mtDNA extend up to 450 km north into the 'Torresian' regions (Shaw and Wilkinson, 1980;Arnold et al, 1987;Marchant, 1988;Marchant et al, 1988;Shaw et al, 1993). While this asymmetrical introgression could be due to selective incorporation of 'Moreton' genes at the hybrid zone, a balance of evidence points to the markers being neutral, and the introgressed genes being the result of southward hybrid zone movement in favour of the 'Torresian' race (Marchant, 1988;Marchant et al, 1988;Shaw et al, 1993) and biased gene conversion .…”

A 2000 km genetic wake yields evidence for northern glacial refugia and hybrid zone movement in a pair of songbirds

“…Less clear is the significance of traits whose clines are shifted far into the geographical range of one of the hybridizing pair. Such shifts may reflect selective introgression of adaptive traits across stationary hybrid zones (Parsons et al 1993;Payseur et al 2004), or genetic wakes of neutral markers left behind following hybrid zone movement (Gyllensten & Wilson 1987;Marchant et al 1988;Rohwer et al 2001). For zones not known to be moving, there has been no formal attempt to test these competing hypotheses.…”

Allozyme variation in Louisiana irises: a test for introgression and hybrid speciation

“…This expectation must now be qualified by the observation that not all genetic markers demonstrate the same propensity to cross reproductive barriers between hybridizing taxa (e.g., see Harrison, 1986;Arnold et a!., 1987;Marchant et al, 1988). The "semipermeable" nature of the boundary between hybridizing taxa (Harrison, 1986) is best explained Note: the relative area that is shaded, unshaded and/or stippled in each of the individual diagrams does not indicate the proportion, but rather, the presence or absence of diagnostic markers for the three species.…”

“…Natural hybridization has been inferred for numerous plant and animal species complexes (e.g., Helianthus, Heiser et a!., 1969; Eucalyptus, Potts and Reid, 1988;Iris, Anderson, 1949; Caledia, Moran and Shaw, 1977;Marchant et a!., 1988;Geomys, Baker et a!., 1989;Menippe, Bert and Harrison, 1988). In addition to describing the occurrence of hybridization, a number of the more detailed studies of natural hybridization between animal taxa have detected the transfer of genetic material between the hybridizing forms (i.e., introgressive hybridization or introgression; Anderson and Hubricht, 1938).…”

“…by the interaction of natural selection, mating behaviour, historical components of the hybridization event, ecological determinants, the genetics of the markers under study, stochastic processes and/or the presence of molecular mechanisms that are capable of increasing the rate and determining the direction of the transfer of certain genomic components (Shaw et a!., 1980;Marchant et a!., 1988;Arnold et aL, 1988;Baker et aL, 1989;Rand and Harrison, 1989;Arnold et a!., 1990). To test the relative importance of these factors on the outcome of hybridization, it is most useful to examine multiple genetic markers for the populations and to compare the distribution of these markers to such things as the ecological context of the hybridizing species, mating systems and modes of reproduction (i.e., sexual and asexual).…”