Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2014 Jul;113(1):74-85.
doi: 10.1038/hdy.2014.5. Epub 2014 Feb 19.

Speciation slowing down in widespread and long-living tree taxa: insights from the tropical timber tree genus Milicia (Moraceae)

K Daïnou  1 , G Mahy  2 , J Duminil  3 , C W Dick  4 , J-L Doucet  5 , A S L Donkpégan  5 , M Pluijgers  5 , B Sinsin  6 , P Lejeune  7 , O J Hardy  8
Affiliations

Speciation slowing down in widespread and long-living tree taxa: insights from the tropical timber tree genus Milicia (Moraceae)

K Daïnou et al. Heredity (Edinb). 2014 Jul.

Abstract

The long generation time and large effective size of widespread forest tree species can result in slow evolutionary rate and incomplete lineage sorting, complicating species delimitation. We addressed this issue with the African timber tree genus Milicia that comprises two morphologically similar and often confounded species: M. excelsa, widespread from West to East Africa, and M. regia, endemic to West Africa. We combined information from nuclear microsatellites (nSSRs), nuclear and plastid DNA sequences, and morphological systematics to identify significant evolutionary units and infer their evolutionary and biogeographical history. We detected five geographically coherent genetic clusters using nSSRs and three levels of genetic differentiation. First, one West African cluster matched perfectly with the morphospecies M. regia that formed a monophyletic clade at both DNA sequences. Second, a West African M. excelsa cluster formed a monophyletic group at plastid DNA and was more related to M. regia than to Central African M. excelsa, but shared many haplotypes with the latter at nuclear DNA. Third, three Central African clusters appeared little differentiated and shared most of their haplotypes. Although gene tree paraphyly could suggest a single species in Milicia following the phylogenetic species concept, the existence of mutual haplotypic exclusivity and nonadmixed genetic clusters in the contact area of the two taxa indicate strong reproductive isolation and, thus, two species following the biological species concept. Molecular dating of the first divergence events showed that speciation in Milicia is ancient (Tertiary), indicating that long-living tree taxa exhibiting genetic speciation may remain similar morphologically.

PubMed Disclaimer

Figures

Figure 1
Figure 1
Principal component analysis (PCA) projection of Milicia individuals based on three leaf traits: L/w_limb (length/width ratio of limb), N_veins (number of secondary veins) and D_veins (distance between two contiguous veins). Black triangles represent samples with Soft=1, a character expected in M. excelsa, whereas red triangles are samples for which Soft=0, a characteristic of M. regia. Country codes: Be, Benin; Ca, Cameroon; Co, Republic of the Congo; Ga, Gambia; Gb, Gabon; Gh, Ghana; Gu, Guinea Bissau et Guinea-Conakry; IC, Ivory Coast; Li, Liberia; Ni, Nigeria; Se, Senegal; Si, Sierra Leone.
Figure 2
Figure 2
Spatial genetic structure of Milicia populations derived from TESS clustering algorithm for an optimal number K=5 clusters. Black crosses indicate morphology-based M. regia (group of) individuals, whereas the white circles stand for M. excelsa. Codes of the 17 sampled countries, from West to East: Se, Senegal; Ga, Gambia; GBi, Guinea Bissau; GCo, Guinea-Conakry; Si, Sierra Leone; IC, Ivory Coast; Gh, Ghana; Be, Benin; Ni, Nigeria; Ca, Cameroon; Gb, Gabon; Co, Republic of the Congo; CK, Democratic Republic of the Congo; Bu, Burundi; Ta, Tanzania.
Figure 3
Figure 3
Geographical distribution of psbA-trnH / trnC-ycf6 (a) and At103 (b) haplotypes in Milicia and median joining networks. Repetitive sequences were included in these analyses.
Figure 4
Figure 4
Phylogenetic trees in Milicia based on (a) pDNA (psbA-trnH and trnC-ycf6 intergenic regions) and (b) nDNA (At103 intron) regions. The tree root height was constrained with the age of the MRCA of Milicia and Morus: mean of 56 mya and s.d. of 9 mya (in millions of years ago (mya)) (Zerega et al., 2005). Numbers at the right part of nodes are posterior probabilities (⩽1). The scale bar at the bottom of each figure indicates time period. The estimated mean ages of selected clades are provided in circles along with their 95% highest posterior density (HPD) interval in brackets (empty brackets: no available HPD information as posterior probability was ⩽0.50). Letters A to K are names used in the text for selected nodes.
Figure 5
Figure 5
A schematic representation of lineage divergence and speciation that reflects the case of Milicia. The circles with cross inside represent a lineage, whereas the empty circles stand for another one. Different gray colors indicate different haplotypes within each lineage and circles are sized according to their relative frequency. Each Milicia cluster (K1 to K5) is mentioned nearby the divergence phase it reaches with respect to the others. From the starting point to phase 1, the two diverging groups inherit the polymorphism of their ancestor and are distinguishable just by allele frequencies. FST=0.1 is obtained after ∼Ne/5 in the absence of homoplasy, Ne being the effective population size. If effective sizes are enough large, mutation will exert more influence than genetic drift on the time to reach phase 2. At this step, some sequences can reveal a monophyletic group, whereas others may show polyphyly. That is the case of the cluster K2 with respect to K3, K4 and K5. Much later, complete extinctions of lineages may occur leading, for example, to one clear monophyletic group and possibly mutual allelic exclusivity (phase 3): this is observed in K1 with respect to the other genetic clusters. Reciprocal monophyly is reached at a later phase (phase 4), after ∼5Ne generations (Rosenberg, 2003): no pair of Milicia clusters displays such a situation.
See this image and copyright information in PMC

References

    1. Anderson EC, Thompson EA. (2002). A model-based method for identifying species hybrids using multilocus genetic data. Genetics 160: 1217–1229. - PMC - PubMed
    1. Anhuf D, Ledru MP, Behling H, Da Cruz FW, Cordeiro RC, Van der Hammen T et al. (2006). Paleo-environmental change in Amazonian and African rainforest during the LGM. Palaeogeogr Palaeocl 239: 510–527.
    1. Armbruster WS, Di Stilio VS, Tuxill JD, Flores TC, Runk JLV. (1999). Covariance and decoupling of floral and vegetative traits in nine neotropical plants: a re-evaluation of Berg's correlation-pleiades concepts. Am J Bot 86: 39–55. - PubMed
    1. ATIBT. (2010). Statistiques 2009. La Lettre de l'ATIBT 32: 4–25.
    1. Aubréville A. (1959) La flore forestière de la Côte d'Ivoire. Deuxième édition révisée—Tome 1. CTFT: Nogent-sur-Marne: Seine (France).

Publication types

Associated data