Modeling Demographics and Genetic Diversity in Ex Situ Collections during Seed Storage and Regeneration

Abstract

Gene banks have a critical role in preserving genetic diversity for many conservation uses. A key challenge in this task is maintaining both the genetic integrity and the viability of a collection during regeneration and long‐term storage. In this study we examine the interacting effects of sampling and viability decline from seed aging on genetic diversity. To do this, we developed a model that simulates sampling during regeneration of individual genotypes using two commonly used but contrasting regeneration protocols. Critically, the model simulates a realistic nonlinear decline in viability across time, and the combined effects of sampling and mortality were evaluated for their effect on allelic diversity. In addition, the model was used to contrast the kinetics of allelic richness among rounds of regeneration in accessions with different heterozygosity levels (e.g., cultivars vs. wild) and different breeding systems (outcrossing vs. inbred). Results underscore a significant interaction between regeneration timing and regeneration size. Regeneration procedures should consider viability kinetics and intra‐accession diversity to best manage genetic erosion. These changes highlight the trade‐off faced by curators between the logistics of increasing regeneration sampling size and improving the technical procedures for seed storage and monitoring.

The Vulnerability of Plant Genetic Resources Conserved Ex Situ

“…Given large volumes of ex situ germplasm, viability testing may also lag behind in under‐resourced genebanks, and many accessions will be older than needed for regeneration in the coming decades. Any further regeneration delay will generate significant loss of unique ex situ genetic diversity, as regeneration delay behaves like a negative truncation selection on germplasm (Richards et al, 2010). Even without regeneration delays, genetic erosion could still occur through genetic drift from the use of inadequate sample sizes for regeneration (Richards et al, 2010).…”

“…Any further regeneration delay will generate significant loss of unique ex situ genetic diversity, as regeneration delay behaves like a negative truncation selection on germplasm (Richards et al, 2010). Even without regeneration delays, genetic erosion could still occur through genetic drift from the use of inadequate sample sizes for regeneration (Richards et al, 2010). More challenging is the regeneration of germplasm accessions for cross‐pollinating species, as it requires additional planning, care, and special techniques to ensure isolation of accessions (Richards, 2001), and thus it is more difficult to maintain germplasm integrity (Chebotar et al, 2003; Krishnan et al, 2013).…”

“…However, inadequate research efforts have been directed toward the efficacy of maintaining and monitoring long‐term seed viability (Fu et al, 2015) and the improvement of germplasm management to minimize within‐genebank genetic erosion (Khoury et al, 2010). Much less attention is paid to research on seed biology under long‐term storage and genetic integrity over long‐term conservation (Li and Pritchard, 2009; Richards et al, 2010). It is important to monitor germplasm viability under long‐term cold storage, like those at the Svalbard Global Seed Vault in Norway.…”

“…Many irreplaceable accessions will be lost with continuous lack of or delayed viability testing and field regeneration with adequate sample size (Qualset and Shands, 2005; Richards et al, 2010). The genetic integrity of a sample will deteriorate over long‐term conservation from untimely or inadequate field regenerations with limited sample size (Brown et al, 1997; Richards et al, 2010) and from long‐term accumulation of mutations (Schoen et al, 1998). The conservation goal for viable germplasm over future generations would be compromised, unless proper measures are developed and acted on in time to mitigate all identified threats.…”

Variance Effective Population Size for Dioecious Species

“…Recently Richards et al (2010) found, by computer simulation, the importance of the interaction between regeneration timing and regeneration size; increasing the sample size is as important as improving the technical procedures for seed storage and monitoring. Gale and Lawrence (1984) discuss this aspect and show that the retention of alíeles within an accession during regeneration is directly proportional to the value of N^.…”