How Discrete are Oak Species? Insights from a Hybrid Zone Between Quercus grisea and Quercus gambelii

Abstract

The white oaks Quercus gambelii and Q. grisea overlap in distribution in New Mexico and Arizona. Within the region of overlap, there are numerous instances of contact between the two taxa. In some areas of contact morphologically, intermediate trees are common, whereas in others, morphologically intermediate trees are rare or absent. We describe a set of RAPD markers that distinguish between the two species and use these markers to examine patterns of gene exchange in an area of contact in the San Mateo Mountains of New Mexico. The markers are highly coincident with morphology and confirm that hybridization between the two species takes place. Despite the occurrence of hybrids, both species remain distinct, even in areas of sympatry, and marker exchange appears to be limited.

Genetic analysis of an ephemeral intraspecific hybrid zone in the hypervariable tree, Metrosideros polymorpha, on Hawai‘i Island

“…In fact, the abundance of morphologically intermediate trees on other intermediate-aged lava flows on east Hawaiʻi Island (all authors, personal observation) suggests that incana-glaberrima hybrid zones form readily on intermediate-aged substrates at lowmiddle elevations on the island. Aging lava flows represent an intermediate, transitional environment between fresh lava fields and mature rainforests where proximity of trees of the two varieties is increased (Howard et al, 1997), the lack of a stable ecological community promotes hybrid community formation (Moore, 1977) and growth and survivorship of intervarietal hybrids may be at least as high as those of the parental varieties in their home environments (Anderson, 1948;Muller, 1952). The high rainfall at low-middle elevations (Giambelluca et al, 2013) should facilitate succession and coexistence of the two varieties and their hybrids (Kitayama et al, 1997).…”

Incipient radiation within the dominant Hawaiian tree Metrosideros polymorpha

“…The evolution of prezygotic reproductive barriers between diverging populations of a tree species is expected to be slow, as evident in the hybridization frequently observed between closely related species (for example, Howard et al, 1997). Weak reproductive barriers in trees are attributed in large part to the tremendous fecundity of trees.…”

Detection of hybrids in nature: application to oaks (Quercus suber and Q. ilex)

“…Interspecific hybridization is the most frequently invoked mechanism to account for the existence of plants morphologically and ecologically intermediate between extant oak species (Jensen et al, 1993;Howard et al, 1997;González-Rodríguez et al, 2004) and to interpret the extensive local sharing of organelle and nuclear genes between species (Whittemore and Schaal, 1991;Howard et al, 1997;Petit et al, 1997;Dumolin-Lapègue et al, 1999). However, in some cases, interspecific gene exchanges have been detected with molecular markers in the absence of obvious morphologically intermediate forms (Whittemore and Schaal, 1991;Dodd and Afzal-Rafii, 2004).…”

“…Moreover, the possibility that shared alleles represent ancestral segregating polymorphisms rather than the outcome of hybridization has been suggested (Muir and Schlö tterer, 2005; but see Lexer et al, 2006). Environmental variation, disturbance as well as the degree of contact between species can affect the frequency and the spatial distribution of hybrids in natural oak populations (Nason, 1992;Rushton, 1993;Howard et al, 1997;Dumolin-Lapègue et al, 1999;Dodd and Afzal-Rafii, 2004;Tovar-Sanchez and Oyama, 2004;Curtu et al, 2007;Valbuena-Carabañ a et al, 2007). Although hybridization between some oak species, such as the closely related species Quercus robur and Q. petraea, has been analyzed extensively for nuclear, chloroplast and mitochondrial variation, our understanding of the underlying processes is still unclear.…”