Airborne‐pollen pool and mating pattern in a hybrid zone betweenPinus pumilaandP. parvifloravar.pentaphylla

Abstract

The reproductive isolation barriers and the mating patterns among Pinus pumila, P. parviflora var. pentaphylla and their hybrids were examined by flowering phenology and genetic assays of three life stages: airborne-pollen grains, adults and seeds, in a hybrid zone on Mount Apoi, Hokkaido, Japan. Chloroplast DNA composition of the airborne-pollen was determined by single-pollen polymerase chain reaction. Mating patterns were analysed by estimating the molecular hybrid index of the seed parent, their seed embryos and pollen parents. The observation of flowering phenology showed that the flowering of P. pumila precedes that of P. parviflora var. pentaphylla by about 6 to 10 days within the same altitudinal ranges. Although this prezygotic isolation barrier is effective, the genetic assay of airborne-pollen showed that the two pine species, particularly P. pumila, still have chances to form F(1) hybrid seeds. Both parental species showed a strong assortative mating pattern; F(1) seeds were found in only 1.4% of seeds from P. pumila mother trees and not at all in P. parviflora var. pentaphylla. The assortative mating was concluded as the combined result of flowering time differentiation and cross-incompatibility. In contrast to the parental species, hybrids were fertilized evenly by the two parental species and themselves. The breakdown of prezygotic barriers (intermediate flowering phenology) and cross-incompatibility may account for the unselective mating. It is suggested that introgression is ongoing on Mount Apoi through backcrossing between hybrids and parental species, despite strong isolation barriers between the parental species.

Direct genotyping of single pollen grains of a self-compatible mutant of Japanese pear (Pyrus pyrifolia) revealed inheritance of a duplicated chromosomal segment containing a second S-haplotype

“…Single-pollen genotyping and sequencing have so far been utilized only in molecular taxonomy and ecological genetics (Zhou et al 2007;Ito et al 2008;Hirota et al 2013). However, we think that singlepollen analysis has the potential for important applications in breeding science, and we expect further utilization of this method.…”

Marking live conifer pollen for long-distance dispersal experiments

“…DNA analysis of a single pollen grain, based on the PCR, is a valued method (Kostia et al 1995;Parducci et al 2005;Ito et al 2008), but it is unsuitable as a live-pollen marking method because it is slow and costly. Another source of marked pollen is pollen that has been genetically modified with recombinant DNA constructs (e.g., Watrud 2004), but this nascent method requires permits and government regulatory oversight.…”

“…Aside from studying LDD, marking live pollen has a number of other landscape-scale and integrative biology applications including: (1) testing LLD theory (i.e., Austerlitz et al 2004;Katul et al 2005), (2) measuring escape from genetically modified (GM) pine plantations (e.g., Kuparinen 2006;Smouse et al 2007;Williams 2010), (3) estimating interspecific hybridization frequency in species complexes during pollination (e.g., Ito et al 2008), and (4) providing an ecologically realistic forecast for how the genetic structure of forests might change under climate change (e.g., Hamrick 2004;Aitken et al 2008). Marking live pollen is pertinent to mating system research, such as detecting pollen-pollen competition, estimating selfing rates at multiple stages (Williams 2008a), and following in vivo pollen-ovule interaction (Takaso et al 1996), and thus it can further forest biology inquiry.…”

Interspecies gene flow is a key evolutionary process for the adaptation of long-lived species to new environmental conditions

“…For instance, a strong assortative mating pattern acting as a prezygotic barrier was discovered in first-generation hybrids between Pinus pumila and P. parviflora, as the combined result of flowering time differences and cross incompatibility (Ito et al, 2008). Genetic introgression between the two species was the consequence of a breakdown of these prezygotic barriers and the occurrence of unselective mating in the subsequent generations through backcrossing between hybrids and parental species (Ito et al, 2008).…”

“…For instance, a strong assortative mating pattern acting as a prezygotic barrier was discovered in first-generation hybrids between Pinus pumila and P. parviflora, as the combined result of flowering time differences and cross incompatibility (Ito et al, 2008). Genetic introgression between the two species was the consequence of a breakdown of these prezygotic barriers and the occurrence of unselective mating in the subsequent generations through backcrossing between hybrids and parental species (Ito et al, 2008). In oaks, as compared to post-zygotic barriers (germination rate and early survival), prezygotic barriers (specifically gametic incompatibility), were shown to be predominantly responsible for reproductive isolation between the species (Lepais et al, 2013).…”