Deleterious mutation accumulation and the regeneration of genetic resources (genetic loadygerm plasmyconservation geneticsylinkage drag)

The accumulation of mildly deleterious mutations accompanying recurrent regeneration of plant germ plasm was modeled under regeneration conditions characterized by different amounts of selection and genetic drift. Under some regeneration conditions (sample sizes >75 individuals and bulk harvesting of seed) mutation accumulation was negligible, but under others (sample sizes <75 individuals or equalization of seed production by individual plants) mutation numbers per genome increased significantly during 25–50 cycles of regeneration. When mutations also are assumed to occur (at elevated rates) during seed storage, significant mutation accumulation and fitness decline occurred in 10 or fewer cycles of regeneration regardless of the regeneration conditions. Calculations also were performed to determine the numbers of deleterious mutations introduced and remaining in the genome of an existing variety after hybridization with a genetic resource and subsequent backcrossing. The results suggest that mutation accumulation has the potential to reduce the viability of materials held in germ plasm collections and to offset gains expected by the introduction of particular genes of interest from genetic resources. Collections of plant and animal germ plasm provide valuable reservoirs of biological materials for use in controlled experiments, for the selective improvement of domesticated species, and as sources of potentially new foods or drugs (1). In the case of plants, the majority of managed germ plasm resources are housed in seed banks. Historically such collections have been associated with agricultural applications, but there is increasing interest in adopting ex situ methods of conservation as part of the overall strategy for the conservation of many noneconomically important species (2). The maintenance of seed viability presents a number of problems for the long-term conservation of plant germ plasm. Although seed preservation technology has been improved in recent years, all stored seeds eventually lose viability. Typically, when germination percentages fall below set limits (65–85% depending on the institution), regeneration of the collection from a finite sample of the stored germ plasm is recommended (1). The frequency with which regeneration must be conducted depends on the type of seeds in question. Plants with so-called ‘‘orthodox’’ seed, that withstand drying and subfreezing temperatures may, in many instances, be stored for decades. Cryogenic preservation in liquid nitrogen (used for a relatively small fraction of the world’s germ plasm resources) can extend storage life for 100 years or more for some species. The seeds of some plants, especially many tropical species, however, cannot tolerate drying and cold storage, and regeneration must be conducted frequently. Moreover, in many developing countries, reliable cold storage facilities are not widely available, and seed banks in these countries often rely more heavily on regeneration (and less on storage) for the maintenance of viable germ plasm. Regeneration of stored seed renders germ plasm collections prone to loss of diversity arising from sampling error (or genetic drift) accompanying the finite size of the adult population used to regenerate the new batch of seed (3, 4). The problem of restricted sample sizes during regeneration is aggravated by pressures imposed on seed banks to manage an ever increasing number of collections (or accessions) with limited funds (5); seed banks often use the smallest number of plants possible for seed regeneration, typically fewer than 100 plants (6). Several practices have been suggested to counteract the genetic changes to the germ plasm collections that may occur during regeneration. These practices include the collection of equal aliquots of seed per plant and the use of optimal growing conditions. Such procedures should maintain effective population size close to its maximum for the number of plants used in regeneration, and minimize selective changes that otherwise might occur in response to the storage and regeneration environment (1). A potential, but largely overlooked, problem accompanying the ex situ storage and regeneration of genetic resources is the accumulation of mildly deleterious mutations. Paradoxically, the same practices that prevent the loss of variation in response to the storage and regeneration environment are also those that allow the number of mildly deleterious mutations to build up in the germ plasm collection. Although mutation accumulation has received attention in relation to mating system evolution and the long-term viability of natural populations (7–12), there have been no detailed analyses of the problem in the context of ex situ genetic resource conservation, though studies by Couvet and Ronfort (13) and Lange (14) suggest that selection against mild deleterious mutations is weak when fertility variation among individuals is minimized. Accumulation of mildly deleterious mutations may lead to loss of viability in germ plasm collections and, in the case of wild species, present obstacles to the use of such seed in any future efforts to reintroduce such materials into natural populations. Mutation accumulation also may complicate the use of such seed in selective breeding. In this paper we use deterministic calculations and simulations to examine how finite sample size and other storage and regeneration conditions may contribute to the accumulation of mildly deleterious mutations in germ plasm collections of self-fertilizing and outcrossing plant species, and we examine some of the consequences of mutation accumulation for the future use of materials stored in ex situ collections. In so doing, we develop a deterministic method for examining mutation accumulation in finite populations of self-fertilizing species. The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked ‘‘advertisement’’ in accordance with 18 U.S.C. §1734 solely to indicate this fact. © 1998 by The National Academy of Sciences 0027-8424y98y95394-6$2.00y0 PNAS is available online at http:yywww.pnas.org. †To whom reprint requests should be addressed. e-mail: Dan_Schoen@ maclan.mcgill.ca.