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Inbreeding can be lethal

Brad Potts
School of Plant Science
University of Tasmania
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Figure 1. Blackberries are a common weed problem in Tasmanian plantations established on ex-agricultural land. Machinery was required to clear blackberries from beneath the inbreeding trial to allow scientists to access the trees in 2005.  (photo: Paul Tilyard).


Inbreeding is one of the key factors which can reduce the productivity of eucalypt plantations, whether they are grown for pulp or carbon.  This message was reinforced recently for E. globulus which has produced the highest level of inbreeding depression yet reported for a eucalypt.

Results from a unique field trial established in 1995 by the CRC for Temperate Hardwood Forestry to quantify the deleterious effects that inbreeding has on the productivity of E. globulus plantations were published recently (Costa e Silva et al. 2010a, 2010b).  Crossing for this experiment and trial establishment was undertaken by Craig Hardner during his PhD studies at UTAS and 10 years of hard-earned data (Figures 1 & 2) were later analysed by Joao Costa e Silva from the University of Lisbon, Portugal during a visit to the School of Plant Science at UTAS. The trial was established on Gunns Ltd. land in northwest Tasmania and was harvested recently.  

As with most eucalypts, E. globulus has a mixed mating system which means that it can both self-pollinate and outcross (Potts et al. 2008).  Average levels of self fertilization in natural populations have been reported to range from 11 to 35% (Mimura et al. 2009), but are likely to be less in well-managed seed orchards (Hodge et al. 1996; Potts et al. 2008).  

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Figure 2.  The space between rows in the inbreeding trial had to be slashed to allow assessors access to trees for measuring stem diameter at breast height. (photo: Paul Tilyard).

We have long known that E. globulus offspring derived from self fertilization or other less severe forms of inbreeding, including natural open-pollination, may exhibit poor growth and survival relative to offspring derived from crossing unrelated individuals (Hardner and Potts 1995; Hardner et al. 1998).  This phenomenon is reported frequently in forest trees and is known as inbreeding depression.  As a result of self-pollination, levels of inbreeding depression in stem volume of up to 48% have been reported.  Previous experiments have involved self-pollinated and open-pollinated families being grown in close competition with more vigorous (controlled) outcrossed families, and this may have accentuated the deleterious effects of inbreeding.  Thus, it is possible that in large plot plantings of different cross types - that are more typical of a plantation situation - such high levels of inbreeding depression would not be translated to such a severe reduction in productivity on a per hectare basis.

The experiment planted in 1995 was unique in that: (i) it had the largest number of base parents (50) yet used in a study of inbreeding in eucalypts; and (ii) it had self-pollinated, open-pollinated and controlled outcrossed (with a mix of 30 pollens) progeny in separate large plots within a replicated field trial to minimize direct competition between cross types. Even in the absence of direct competition, after ten years the inbreeding depression for survival (74%; Figure 3) and stem basal area per hectare (78%) in the self-pollinated progeny were the highest yet reported for a eucalypt species.


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Figure 3.  Percentage survival over a 10 year period of controlled outcrossed, open-pollinated and selfed families of E. globulus grown in separate plots in a replicated field trial in northwest Tasmania (data from Costa et Silva et al. 2010a).


Even the vigorous outcrossed individuals among the open-pollinated progeny did not grow sufficiently better than the self-pollinated progeny to compensate for the deleterious effects of inbreeding; the basal area per hectare in open-pollinated progeny plots was still 32% less than that of the progeny of controlled outcrossing.  It was also interesting to note that the stems in the controlled outcrossed plot were relatively uniform in size compared to the open-pollinated and self-pollinated plots.  We estimated that the original outcrossing rate in the planted open-pollinated seedlings was approximately 50-60% and this increased with age as self-pollinated progeny died. 






References


Costa e Silva J, Hardner C, Tilyard P, Pires A and Potts BM (2010a) Effects of inbreeding on population mean performance and observational variances in Eucalyptus globulus. Annals of Forest Science (read online if you have access to this journal).

Costa e Silva J., Hardner C, Potts BM (2010b) Genetic variation and parental performance under inbreeding for growth in Eucalyptus globulus. Annals of Forest Science (read online if you have access to this journal).


Biobuzz issue twelve, August 2010