Hybrid vigor - Isn't it all relative?

It seems that we need some way to quantitate the degree of outbreeding, because there is probably some optimum degree of outbreeding that may differ between organisms and between traits.

Unraveling the genetic basis of hybrid vigor

James A. Birchler, Hong Sao, Sivanandan Chudalayandi

PNAS 103, 12957-12958 (August 29, 2006)

Abstract: The study of hybrid vigor and inbreeding depression traces back to Charles Darwin, who was the first scientist to examine the phenomenon in a systematic manner (1). Hybrid vigor, or heterosis, is the increase in stature, biomass, and fertility that characterizes the progeny of crosses between diverse parents such that the F₁ is superior to the better of the two parents. In plants, this is basically achieved by a greater proliferation of cells in some but not all tissues (2). Inbreeding depression refers to the decline in the quantitative measure of these characters upon self-fertilization or other forms of homozygosis of alleles (inbreeding). The genetic basis of heterosis has been debated for nearly a hundred years without an emerging consensus (3–5) (Fig. 1). An early view was that the combination of different alleles in an organism resulted in a superior state for growth and vigor compared with the presence of identical alleles (3). As genetic knowledge increased, the concept that inferior alleles of different genes in the two parents were complemented in the hybrid (6), thus leading to the superior characteristics, gained favor. Although the latter explanation is simple and easily envisioned, results that seemed to favor interactions of diverse alleles have been repeatedly found. In this issue of PNAS, the work of Semel et al. (7) examined an extensive set of quantitative traits in partial hybrids of domesticated tomato and a wild relative. They conclude that most traits that exhibit heterosis do so as a result of heterozygosity of the controlling genomic regions to produce traits superior to the better parent. They also suggest that heterosis was selected over evolutionary time for characteristics that impact reproductive success.