FAQ: Why should I try to sample from 50 maternal plants?
The 50 maternal lines recommendation is supported by the sampling strategy from the Bureau of Land Management Technical Protocol for the Collection, Study, and Conservation of Seeds from Native Plant Species for Seeds of Success (2016).
For many potential users of and uses for the collection, it is important to maximize the number of alleles (variants of genes) present within the sample by capturing the greatest proportion of those alleles represented in the field population. The number of different alleles in a population reflects its genetic diversity. Sampling from (1) 30 randomly chosen individuals in a fully outcrossing, or outbreeding, sexual species, or (2) 59 randomly chosen individuals in a self-fertilizing species will capture at least one copy of 95% of the population’s alleles which have frequencies of at least 0.05 (Brown and Marshall 1995).
This analysis suggests that, with care, a single population seed sample collected in this way would possess the potential for re-establishment at that site, and perhaps for establishment at other sites within the natural range of the species.
The reproductive biology of most target species has not been studied, and the capture of very rare alleles would require a markedly increased sample size, so collectors are advised to sample from an excess of 50 individuals growing together in a single population where available and to look for populations with a large number of plants.
For research purposes, and for the conservation of rare species that occur in populations of fewer than 50 individuals, as well as for less fecund common species, where collections will result in fewer than 3000 seeds, we recommend collecting seeds along maternal lines. In a maternal line collection, seeds from each individual plant (maternal line) are collected and bagged separately (as opposed to bulking the seed collection of multiple plants into one or two bags as we do for regular seed bank collections). This ensures the greatest genetic diversity is available in a small collection. When we combine seeds in a bulk collection, there is only a small chance any particular parent’s offspring will be represented when a portion of the collection is removed to restore a new population. For small populations or small seed collections, collecting along maternal lines allows equal representation of all parent’s offspring (seeds) in a newly restored population, provided a portion of the seeds are distributed equally from each maternal line (Bureau of Land Management 2016).
To answer this question we need to pay closer attention to the number of populations available for sampling, the likely current and historic geneflow between populations, and the expected future use of the seed collections. Lets not make this more complicated than necessary, but we do need to bring some species information into the equation to be able to estimate better how many individuals should be sampled from, and from how many populations. I am looking out for some recent modelling by Sean Hoban to give us some boundaries to improve current guidance. Michael.
The overall answer is correct- that seed from 50 maternal plants will represent the genetic diversity of one target population (note- not the whole species). This assumes the target is common alleles, those above 0.05 frequency. For some rare alleles, more samples may be needed, but that is the justification for the 50 maternal plant threshold. It is important to think about the role of the ex situ collections- for augmentation, for restoration, for breeding, or for preventing extinction.
An ex situ population size above 50 individuals will also help avoid inbreeding in the short term (but not long term)
If there are many populations (say, 5, 10 or 50 populations), you could sample from 50 individuals from every site (for example, a total of 250, 500, or 2500 maternal lines), but you could possibly sample less. As Michael says, it depends a bit on the number of populations and how isolated they are. If possible try to visit each population, especially any isolated populations or those in unusual environments which might have local adaptations. Based on my best knowledge at this point, sampling seed from somewhere between 100 and 1000 total in situ individuals is needed to represent genetic diversity ex situ across a metapopulation of a rare species, spread across the populations. This would be a good standard for a species-wide conservation collection of a rare plant.
Be sure to collect enough seed to allow for losses along the way- seedlings die etc., so you want to collect enough so you have enough viable seeds.