Paternity analysis

Title

• Title: Determination of Pollen flow in E. camaldulensis Dehnh. (River Red Gum) clonal seed orchard by paternity analysis
• Target: Tree Genetics & Genomes
• Authors: Rathinavelu R, Vishwanath M, Rathinam K and Varghese M

Introduction

Eucalyptus camaldulensis Dehnh. (River Red Gum) is the introduced and most widespread eucalypt, occurring in India. Eucalypts are raised extensively as pulpwood plantations and the prime objective of an improvement program is to enhance wood and pulp yield. Improvement through clonal forestry of superior types takes the advantage of immediate gain, however handicapped with the progressive gain in the subsequent generations. Seed orchards have proven to be the better strategy of maintaining the breeding population for the sustainable improvement, where the genetic gain is slow, but progressive.

In seed orchard, the differential family performance has been attributed to possible inbreeding depression resulting from selfing and/or mating between relatives (Hodge et al., 1996). Direct comparisons of growth rates of self pollinated and out crossed eucalypts have demonstrated negative effects of inbreeding on seed viability and growth (Hodgson, 1976; Grigffin and Cotterill, 1988; Hardner and Potts, 1995). A positive association between outcrossing rate and growth has also been reported in many Eucalypt species. The differences in growth and survival among families and populations of E. camaldulensis raised the question of whether this variation could be explained by differences in outcrossing rates.

Despite extensive plantings of E. camaldulensis in India and the establishment of breeding programs, reports of outcrossing rates in this species have been limited. The species is hermaphrodite and insect pollinated with bees, both native (Trigona spp.) and honey bees (Apis mellifera), the main pollinators. In common with other eucalypts, E. camaldulensis has a mixed mating system. Individual flowers are protandrous, but geitonogamous pollinations can occur as flowers on the same tree develop at different times. The outcrossing rate for seed crops from individual trees is influenced by a variety of genetic and environmental factors including self-incompatibility, flowering phenology relative to neighboring trees in the stand, heterogeneity in the pollen pool and ecological factors, which influence the pollinator activity.

Outcrossing rates determine the coefficient of relationship, which is used in calculations of heritability and expected genetic gain from open-pollinated progeny trials. Reports of moderate levels of inbreeding in eucalypts led to suggest that genetic improvement may be less than expected using recurrent selection from open pollinated seed orchards. Inbreeding would reduce the number of new combinations of genotypes on which future genetic gain by selection depends, whether from seed orchards or plantations. In addition, when populations are partially inbred and exhibit inbreeding depression for fitness components, the quantitative genetic parameters estimated may not reflect the evolutionary potential of these populations. Additive genetic variance will be over-estimated if the parental population is structured (leading to heterogeneity in the pollen pool), or there is a substantial degree of inbreeding. This, in turn will bias estimates of heritability and genetic gain.

Mating patterns, including pollen dispersal and gene flow within and between populations, are important parameters for understanding the genetic structure of plants. In addition to their importance to population genetics and evolutionary biology, patterns of gene movement by pollen dispersal are of practical significance to forestry (Adams et al. 1992). Knowledge of mating patterns in natural populations is important for assessing the genetic composition of seed collected from natural stands for inclusion in breeding programs (Kitzmiller 1990) and for formulation of gene conservation strategies including number and distribution of parent trees for seed collections or distances required to isolate forest preserves (Ledig 1986).

There are many methods proposed to study the outcrossing rate in a population in which, Deoxyribonucleic acid (DNA) polymorphisms provide a powerful tool for quantifying the existing levels of outcrossing in breeding and production populations of forest trees. Several molecular marker technologies are available today, to perform the paternity analysis, of which simple sequence repeats (SSR) has proven to the most appropriate marker system. SSRs are also known as microsatellites, is certainly the most efficient way of resolving issues of identity. SSRs are typically co dominant and multiallelic, with expected heterozygosity frequently greater than 0.7, allowing precise discrimination even of closely related individuals. Due to the specificity of the PCR assay and its high information content, it also allows the determination of identity between individuals based on formal estimates derived from allele frequencies. As microsatellite marker information can be easily shared between laboratories based on primer sequences, inter laboratory comparisons of data are straightforward, improving co-operative efforts in the standardization of data. SSRs have been the most practical marker system to determine the outcrossing rate in the population, unfortunately there is no published data on the development of SSR markers in E. camaldulensis.

In the present investigation, we have estimated the out crossing rate of 8 selected individuals from clonal seed orchard using SSR markers to ascertain the reproductive success of selected individuals

Keywords

Eucalyptus camaldulensis, mating system, outcrossing rate, microsatellite markers

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