DTA

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Tesi etd-03292019-164028

Type of thesis
Dottorato
Author
MCLEAN RODRIGUEZ, FRANCIS DENISSE
URN
etd-03292019-164028
Title
Comparison of in situ and ex situ maize agrobiodiversity conservation through genomic and socioeconomic approaches. A case study in Morelos, Mexico
Scientific disciplinary sector
AGR/07
Course
SCIENZE AGRARIE E BIOTECNOLOGIE - Agrobiodiversity
Committee
relatore Prof. PE', MARIO ENRICO
Membro Prof. GIANFRANCESCHI, LUCA
Membro Prof. BINELLI, GIORGIO
Membro Prof.ssa FRASCAROLI, ELISABETTA
Membro Dott.ssa MOONEN, ANNA CAMILLA
Keywords
  • native maize
  • genetic diversity
  • genetic erosion
  • plant genetic resources
  • Zea mays
Exam session start date
;
Availability
parziale
Abstract
The conservation of crop genetic resources is necessary at all levels, including crop races, landraces and genes, to guarantee the sustainability of traditional and modern agricultural systems. Breeders and farmers conserve crop agrodiversity using two complementary strategies. Ex situ populations are kept under long-term storage conditions, while farmers conserve in situ populations in their fields. To design effective conservation strategies, it is essential to understand how populations conserved in the field can differ with time from populations conserved in genebanks. Because a complex range of socioeconomic factors influence in situ conservation, this understanding is necessarily achieved through a combination of genomic and socioeconomic approaches.<br><br>In this longitudinal study, we used a collection of 93 maize landrace accessions from Morelos, Mexico, and stored at the International Maize and Wheat Improvement Center (CIMMYT) Maize Germplasm Bank since 1967, to trace back to the original 66 donor families after 50 years and explore the causes for why they abandoned or conserved their seed lots. To compare ex situ and in situ conservation at the race and landrace level, an actor-centered approach was used based on interviews and focus group discussions. A Multi-Level Perspective framework was adopted to examine loss as a process, accommodating multiple causes and the interactions among them. <br><br>We found that the importance of maize landrace cultivation had diminished over the last 50 years in the study area. By 2017, 13 families had conserved a total of 14 seed lots directly descended from the 1967 collection. Focus group participants identified 60 accessions that could still be found in the surrounding municipalities. Findings showed that multiple interconnected changes in maize cultivation technologies, as well as in maize markets, other crop markets, agricultural and land policies, cultural preferences, urbanization and climate change, have created an unfavorable environment for the conservation of maize landraces. Many of these processes were location- and landrace-specific, and often led to landrace abandonment during the shift from one farmer generation to the next.<br><br>To compare ex situ and in situ conservation at the genetic level, a comparison of the genetic diversity between a set of 13 ex situ accessions from the 1967 collection and another set of 13 in situ accessions retrieved from the same donor-families in 2017 is presented. Few studies retrieve in situ accessions from the same farmers who donated ex situ accessions, which would be the closest method to ensure sampling the same seed lots. For genotyping, 74,739 SNPs were obtained using ddRAD sequencing from 10 individual seedlings per each accession (260 seedlings total). Genetic diversity indicators, population structure and signals for selection were compared between ex situ and in situ accessions. Information from farmers’ interviews and focus group discussions was used to identify potential sources of gene flow and farmers’ seed selection criteria. <br><br>We found that in situ accessions together had the same expected heterozygosity and average minor allele frequency as ex situ accessions. However, they had a lower number of SNPs due to the disappearance of rare and low frequency alleles. Few changes in common alleles were detected. Single accessions followed different trajectories in farmers’ fields. Four accessions increased their similarity with other accessions in the panel while increasing their diversity. Three accessions differentiated from the panel while reducing their diversity. Both processes were likely related to pollen flow between landraces of adjacent plots. However, despite the identified changes, all accessions studied constituted a single population from a genome-wide perspective. We found evidence for selection for overall ear fitness traits between all ex situ and in situ accessions, but the strongest signal was found between ex situ and in situ Ancho accessions. After 50 years, genebanks have successfully conserved the largest share of the genetic diversity of this maize population. Meanwhile, farmers have conserved or enhanced the genetic diversity of most seed lots while they continue with the selection that has shaped maize morphological diversity.
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