PREMISE: A good understanding of genetic variation and gene dispersal in tree populations is crucial for their sustainable management, particularly in a context of rapid environmental changes. West African Sudanian savannahs are undergoing fragmentation and degradation, partly due to expansion of crop cultivation and monocultures that reduce tree density and may impact pollinators. This could affect the population dynamics of important indigenous trees. We investigated the influence of habitat fragmentation on patterns of genetic diversity and gene dispersal of a key Sudanian agroforestry tree species, Parkia biglobosa.

METHODS: Using ten highly polymorphic nuclear microsatellites, we genotyped 2475 samples from reproductive trees, seedlings and embryos in four tree populations presenting different levels of habitat fragmentation.

RESULTS: Parkia biglobosa presented high genetic diversity, similar across the four populations studied. Genetic diversity and inbreeding were similar between adults and embryo cohorts. In all four populations the selfing rate was less than 1%. A high effective number of pollen donors per tree (NEP~18-22) and a high pollen immigration rate were observed (ranging from 34 to 74%). Pollen dispersal was characterized by a fat-tailed distribution with mean estimates exceeding 200 m. In three populations, stem diameter had a pronounced effect on male reproductive success. Here, the highest male reproductive success was observed in trees with a diameter at breast height of between 60 and 75 cm.

CONCLUSIONS: At the scale analyzed, fragmentation does not seem to pose limitations to gene flow in any of the sites investigated, regardless of the landscape configuration associated with the different tree stands examined. The study provides useful insights on the reproductive biology of an important tree species in the West African savannahs.

Overall, DNA was isolated from seed and leaflet samples of 2475 individuals composed of 703 adult trees (167 small and 536 large trees), 1747 embryos and 25 seedlings from Cassou and Saki. The total genomic DNA was extracted using the DNeasy 96 Plant Kit protocol (QIAGEN, Hilden, Germany), following the manufacturer’s instructions. DNA samples were genotyped using 10 nuclear microsatellite loci (nSSR) developed by Lassen et al. (2014) and following the method applied in Lompo et al. (2018).

Lassen, K. M., Kjær, E. D., Ouedraogo, M. and Nielsen, L. R. 2014. Microsatellite primers for Parkia biglobosa (Fabaceae: Mimosoideae) reveal that a single plant sires all seeds per pod. Applications in Plant Sciences 2: 1-4.

Lompo, D., Vinceti, B., Konrad, H., Gaisberger, H., Geburek, T. 2018. Phylogeography of African locust bean (Parkia biglobosa) reveals genetic divergence and spatially structured populations in West and Central Africa. Journal of Heredity 109: 811-814.

Column names denote:

Running No; sampleID; Population name; Marker data for 10 nuclear microsatellites (PbL03, PbL05, PbL22, PbL15, PbL04, PbL12, PbL11, PbL02, PbL09, PbL21) in “allele1-allele2” form; UTM coordinates and UTM zone (X_UTM, Y_UTM, Zone); DBH (cm); Total height (m); Crown height (m); Crown diameter (m); Tree location (Plot or Transect); Cohorts (Large Tree, small tree, seedling, embryo); Maternal tree (+ = yes).

Lines 2-721 denote sampled trees

Lines 723-2468 are embryos extracted from seed from mother trees (names are given as number of mother tree plus running number of seed).