Maximizing genetic representation in seed collections from populations of self and cross-pollinated banana wild relatives

Abstract

Background Conservation of plant genetic resources, including the wild relatives of crops, plays an important and well recognised role in addressing some of the key challenges faced by humanity and the planet including ending hunger and biodiversity loss. However, the genetic diversity and representativeness of ex situ collections, especially that contained in seed collections, is often unknown. This limits meaningful assessments against conservation targets, impairs targeting of future collecting and limits their use. We assessed genetic representation of seed collections compared to source populations for three wild relatives of bananas and plantains. Focal species and sampling regions were M. acuminata subsp. banksii (Papua New Guinea), M. balbisiana (Viet Nam) and M. maclayi s.l. (Bougainville, Papua New Guinea). We sequenced 445 samples using suites of 16–20 existing and newly developed taxon-specific polymorphic microsatellite markers. Samples of each species were from five populations in a region; 15 leaf samples from different individuals and 16 seed samples from one infructescence (‘bunch’) were analysed for each population. Results Allelic richness of seeds compared to populations was 51, 81 and 93% (M. acuminata, M. balbisiana and M. maclayi respectively). Seed samples represented all common alleles in populations but omitted some rarer alleles. The number of collections required to achieve the 70% target of the Global Strategy for Plant Conservation was species dependent, relating to mating systems. Musa acuminata populations had low heterozygosity and diversity, indicating self-fertilization; many bunches were needed (> 15) to represent regional alleles to 70%; over 90% of the alleles from a bunch are included in only two seeds. Musa maclayi was characteristically cross-fertilizing; only three bunches were needed to represent regional alleles; within a bunch, 16 seeds represent alleles. Musa balbisiana, considered cross-fertilized, had low genetic diversity; seeds of four bunches are needed to represent regional alleles; only two seeds represent alleles in a bunch. Conclusions We demonstrate empirical measurement of representation of genetic material in seeds collections in ex situ conservation towards conservation targets. Species mating systems profoundly affected genetic representation in seed collections and therefore should be a primary consideration to maximize genetic representation. Results are applicable to sampling strategies for other wild species.

Phylogeography and conservation gaps of Musa balbisiana Colla genetic diversity revealed by microsatellite markers

“…The mating system of wild banana species is an important indicator to optimise seed sampling strategies. For (partially) cross-pollinated species such as Musa balbisiana , increasing the number of sampled populations is more effective in maximizing genetic capture than sampling more mother plants per population (Kallow et al 2021 ) though within-species variation in mating system should be taken into account. To further maximize genetic capture, large populations distant from anthropogenic activities should be prioritised (Andersson and de Vicente 2010 ; Almeida-Rocha et al 2020 ; Kallow et al 2021 ).…”

“…More recently, two studies used a set of 18 polymorphic SSR markers to interlink genetic variation in wild M. balbisiana accessions with that in ex situ collections. Bawin et al ( 2019 ) compared the genetic diversity of seeds from wild M. balbisiana populations from Yunnan (China) with those from ex situ seed collections, whereas Kallow et al ( 2021 ) compared seed collections to source populations of M. balbisiana . In the present study, we investigated the genetic structure of M. balbisiana in its complete distribution range (native + feral) in Southeast Asia and Melanesia.…”

Banana seed genetic resources for food security: Status, constraints, and future priorities

“…Additionally, in our gap and conservation assessment we used GRSex and ERSex as well as taxa representativeness, importantly, these are only proxies for genetic representativeness. Assessment of genetic diversity in seed collections has been employed for only a few ex situ seed collections (Gargiulo et al, 2019 ; Wei & Jiang, 2020 ), and three Musa taxa (Bawin et al, 2019 ; Kallow, Panis, et al, 2021 ). These can be used as the basis of improving sampling, such as defining how many seeds or mother plants should be collected to best capture genetic diversity present in populations, and where collections should be targeted to sample uncollected alleles with optimal efficiency.…”

“…Models were then cropped according to species geographic distributions described by Govaerts and Häkkinen ( 2006 ). We buffered each seed collection location by a 50 km radius; and for species known to self‐pollinate indices were also calculated with a 5 km buffer, because self‐pollination restricts representativeness of Musa seed collections (Kallow, Panis, et al, 2021 ). The third index was an ecological representativeness score (ERSex).…”

“…Values for FCSex were then categorized as follows in the same manner as Khoury et al ( 2019 ): <25 high priority (HP), ≥25 <50 medium priority (MP), ≥50 <75 low priority (LP), ≥75 sufficiently conserved (SC). We used all seed collections in the analysis, even those without viability estimates or those recorded as 0% viability, because only a very few viable seeds can represent the genetic diversity in Musa seed collections (Bawin et al, 2019 ; Kallow, Panis, et al, 2021 ), and we assumed that such low numbers may be present even in these samples. A total of 513 seed accessions of 53 taxa and 2079 occurrence records were used in the gap analysis and conservation prioritization (excluding accessions not defined to species level); of which 274 seed accessions of 26 taxa had unique coordinates and SDMs (used to calculate GRSex and ERSex).…”