Population Genetics and Phylogeny of European Blackberries

Abstract

Abstract

European blackberries constitute a polyploid complex of closely related microspecies, which includes species in Rubus sect. Rubus and Rubus sect. Corylifolii of Rubus subgenus Rubus. These species reproduce by facultative agamospermy. New microspecies evolve from time to time by hybridization between Rubus sect. Rubus and R. caesius or R. idaeus, or by sexual reproduction. Because of their almost inexhaustible variation, new microspecies are continuously being described, but it is often unclear whether disjunct populations of similar morphology from different regions belong to the same microspecies, or have evolved independently from each other. In this study, I used codominant markers, nuclear microsatellites (or simple sequence repeats), to investigate their gene diversity. I also studied the two ancestral species R. caesius and R. ideaus. A correlated allele frequency distribution pattern was observed between these two species and members of the blackberries. Polyploid blackberries had a high gene diversity and low differentiation among populations, which are probably due to their relatively high level of sexual reproduction as compared to agamospermy. Various descriptors of genetic distance were used to calculate gene diversity, Ds, (δ*)2, DA and D′. The phylogenetic tree based on D′, which is a modification of Ds, separates R. ideaus from subgenus Rubus and puts sect. Rubus and sect. Corylifolii as sister groups. R. caesius and R .ideaus were more distantly
related. In general, D′ resulted in the most reasonable tree among all four parameters.

Popular Abstract

European Blackberries and Agamospermy

European blackberries constitute a polyploidy complex of related microspecies, which includes species in Rubus sect. Rubus and Rubus sect. Corylifolii of Rubus subgenus Rubus. Polyploid blackberries mainly reproduce by agamospermy and sometimes by sexual, which are called facultative agamospermy. Agamospermy is clonal reproduction by means of seed, which provides offspring with advantages, like dormancy, protection and dispersal, over offspring from vegetative reproduction. The essential elements of agamospermy are non-reductional meiosis, avoidence of egg fertilization, and development of endosperm (with or without fertilization). Agamospecies usually have close association with polyploidy and hybridization. They have advantages of fixed heterosis, masking negative mutations, escaping from sterility, etc. On the other hand, their disadvantages are restricted sexual recombination, accumulation of deleterious mutation, etc. Once established, agamospecies colonize available habitats rapidly, but their evolutionary potential is limited unless they are possible to perform sexual recombination.

Blackberries need to fertilize endosperm nuclei to form a functional endosperm (pseudgamy), which request reserves blackberry’s pollen viability in some degree. So, agamospermous blackberries can act as pollen donor to cross with sexual relatives, Rubus. ideaus and R. caesius. R. ideaus, from subgenus Idaeobatus, is diploid and reproduces sexually. R. caesius is tetraploid, which belongs to subgenus Rubus with its own section Caesii, and is characterized by facultative agamospermy. It can easily hybridize with sect. Corylifolii.

There are five known diploid species in Rubus sect. Rubus that worked as the phylogenetic foundation of polyploidy blackberries in Europe. Some diploids from North America have also contributed to the European blackberries. Polyploids in sect. Rubus are produced by fertilizing of unreduced gametes of diploid species and facultative agamospermy of polyploid species of sect. Rubus. Whereas the sect. Corylifolii species are believed to have arisen from hybrids between R. caesius (dewberry) and various species of sect. Rubus. R. ideaus can also form hybrids with polyploids in sect. Corylifolii. Those hybrids, as well as hybrids from back-crosses and intercrosses in sect. Corylifolii, are the main cause of the overwhelming variation and taxonomical problems observed among the Swedish sect. Corylifolii species.

Because of their almost inexhaustible variation, new microspecies are continuously being described, but it is often unclear whether disjunct populations of similar morphology from different regions belong to the same microspecies, or have evolved independently from each other. In this study, I used codominant markers, nuclear microsatellites (or simple sequence repeats), to investigate gene diversity of some polyploid blackberries, R. caesius and R. ideaus. A correlated allele frequency distribution pattern was observed between these two species and members of the blackberries. Polyploid blackberries had a high gene diversity and low differentiation among populations, which are probably due to their facultative agamospermy reproduction system and wide dispersal of clonal seeds.

Various descriptors of genetic distance were used to calculate gene diversity, Ds, (δμ)2, DA and D′. The phylogenetic tree based on D′, which is a modification of Ds, separates R. ideaus from subgenus Rubus and puts sect. Rubus and sect. Corylifolii as sister groups. And R. caesius were more distantly related to R. ideaus and sect. Rubus than to sect. Corylifolii. In general, D′ resulted in the most reasonable tree among all four parameters.

Supervisor: Mikael Hedrén
Master´s Degree Project in Plant Ecology and Systematics 45 credits, 2014
Department of Biology, Lund University

• author: Zhou, Mingyue
• supervisor: Mikael Hedrén
• organization: Degree Projects in Biology
• course: BION31 20132
• year: 2014
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 4675509
• date added to LUP: 2014-09-24 12:00:49
• date last changed: 2014-09-25 14:21:02