Fine-scale estimation of outcrossing in western redcedar with microsatellite assay of bulked DNA

Abstract

Western redcedar (Thuja plicata, Cupressaceae) is a selffertile conifer with a mixed mating system and significant variation for outcrossing among populations. In this paper, we conducted a fine-scale study of mating system variation to identify correlates of outcrossing in natural populations. We examined variation for outcrossing within and among individual trees, and describe a new method to estimate outcrossing using bulked DNA samples. Bulking (assaying DNA tissues from several individuals simultaneously) increases the experimental power without increasing the experimental effort. We sampled 80 trees from four natural populations in southwestern British Columbia. From each tree, we sampled from up to six crown positions (three heights and inner vs outer branches). From each position, two samples of three seedlings each were bulked before DNA extractions. Using four microsatellite loci, we obtained outcrossing rates for each tree and for each of the six crown positions. We found individual tree selfing rates to increase with tree height in all four populations, but selfing rates did not differ among crown positions. The higher selfing rate of larger trees is probably due to their greater proportional contribution to local pollen clouds. Individual tree outcrossing rates ranged from 22 to 100% and the population outcrossing rates from 66 to 78%. Missed alleles due to bulking and the estimation method used both cause a downward bias in outcrossing rates, so that these estimates are probably lower than the actual outcrossing rates. Nevertheless, the trends we observed are not affected by systematic biases of estimation.

Mating system and pollen gene flow in Mediterranean maritime pine

“…The positive relationship between tree and crown size and outcrossing rates suggested that the proportion of outcrossed pollen reception might be enhanced in bigger trees with ample crowns, decreasing selfing rates. However, O'Connell et al (2004) observed that outcrossing rates decreased with increasing tree height in four populations of Thuja plicata. This variety of results emphasizes the idea that tree size, apart from directly affecting outcrossing rates through increasing flower exposition to the pollen cloud, may also be a gauge for other factors affecting mating patterns, such as reproductive effort or tree social position within the stand (Burczyk et al, 1996;González-Martínez et al, 2006).…”

“…At the stand and population level, mating patterns are affected by natural factors such as population density and size, mode of pollination, flowering synchrony and phenological patterns (El-Kassaby and Jaquish, 1996;Robledo-Arnuncio et al, 2004a;Gérard et al, 2006;O'Connell et al, 2006), as well as management and silvicultural practices (Murawski et al, 1994;El-Kassaby et al, 2003). At the individual level, plant size and shape (in particular crown shape in trees), gender expression, male and female reproductive effort and within-stand social position can also affect mating patterns (Burczyk et al, 1996(Burczyk et al, , 2002Dow and Ashley, 1998;O'Connell et al, 2004;García et al, 2005;Verdú et al, 2006). Forest trees typically have a low level of domestication, and management strategies in forest tree breeding programs, in situ conservation units or ex situ conservation plantations often rely on open pollination.…”

Morphological and physiological variation in western redcedar (Thuja plicata) populations under contrasting soil water conditions

“…Individual trees were randomly sampled in each stand. Parental trees, which had the potential of being pollinated by numerous males (Schuster and Mitton 2000;O'Connell et al 2004), were separated by a minimum of 200 m, and 500 m on average, to minimize relatedness among females. In addition, cones were collected from the upper crown only, potentially minimizing the selfing rate (O'Connell et al 2001).…”

Impacts of forest fragmentation on the mating system and genetic diversity of white spruce (Picea glauca) at the landscape level

“…Nevertheless, in wind-pollinated conifers, the mating system can be affected by population structure attributes. For example, in some conifer species, outcrossing rates are correlated with population density (Farris and Mitton, 1984;Rajora et al, 2002), family structure and tree heights (Mitton, 1992;O'Connell et al, 2004). In wind-pollinated trees, surrounding vegetation can also impede pollen dispersal, so that fragmentation can increase pollen and gene flow distances between forest trees (Young and Merriam, 1994;Dyer and Sork, 2001).…”