Evolution of agamic complexes

Eastern North American Amelanchier is an agamic complex (Campbell and Wright 1996), in which the combination of apomixis, polyploidy, and hybridization creates complex patterns of diversification. Understanding the evolution and advancing the systematics of Amelanchier requires an appreciation of how diversification occurs in agamic complexes. Circumscription of species, which is particularly challenging in agamic complexes, is considered on the ‘Systematics’ page.

In apomixis, egg-bearing megagametophytes form without meiosis (apomeiosis), and the egg develops into an embryo without fertilization (parthenogenesis; Savidan 2007). In Amelanchier apomicts, when meiosis is unsuccessful (meiosis is sometimes successful in facultative apomicts; see below), the megasporocyte or megaspores degenerate into a mass of tissue (see top photo on the left). At the same time, chromosomally unreduced nearby cells (aposporous initials) can develop into megagametophytes. The megasporangium in the top photo on the left is interpreted as having a two-nucleate megagametophyte to the right of the degenerating mass of tissue. Furthermore, there is usually more than one megagametophyte in an ovule, presumably from the maturation of more than one aposporous initial or of aposporous initials plus megaspores. The overall apperance of Amelanchier megametophytes that are apomeiotic differ from those that are the product of meiosis only in fusion of the two polar nuclei in mature meiotic megagametophytes. A megasproangium with two mature megagametophytes, each with unfused polar nuclei and therefore presumably apomeotic, is shown in the top image on the right. For more information about the embryological details of apomixis in Amelanchier, see Campbell et al. (1985, 1987) and Campbell and Wright (1996).

Apomixis has been documented in 35-40 families and about 130 genera of flowering plants, about 70% of which are from Asteraceae, Poaceae, and Rosaceae (Carman 1997). There is considerable variation among taxonomic groups in the genetic control of apomixis, and the nature of this control is not fully understood Carman 2007, Curtis and Grossniklaus 2007, Vijverberg and Van Dijk 2007). The occurrence of apomixis in the offspring of a cross between a sexual and an apomictic species (Weber and Campbell 1989) suggests that apomixis is dominant to sexuality in Amelanchier.

Apomicts, including those in Amelanchier, are polyploid, with very few exceptions (Savidan 2007). Polyploidy is often accompanied by rapid genomic changes and massive gene silencing of duplicated genes (Adams and Wendel 2005). Polyploids often have more heterozygosity, genetic diversity, biochemical diversity, selfing, and colonizing ability than their diploid progenitors (Soltis and Soltis 2000, Soltis et al. 2003). Polyploidization has large impacts on taxonomic diversity, being credited with 2-4% of all speciation events in angiosperms (Otto and Whitton 2000). Recurrent formation of polyploids is considered the rule, introducing polyphyly into phylogenies (Soltis et al. 2003, Tate et al. 2005). While allopolyploidy is more common than autopolyploidy, the latter is thought to be much more common than it formerly was and can lead to speciation (Soltis et al. 2007).

Most apomicts are facultative, producing both apomictically and sexually, and can hybridize with other taxa. Indeed, hybridization is common in agamic complexes, including Amelanchier (Gustaffson 1946-1947, Campbell and Wright 1996, Bayer and Chandler 2007, Fehrer et al. 2007, Noyes 2007, Weber and Campbell 1989). Apomicts are usually the pollen parent in matings with sexuals because most apomicts, including those in Amelanchier, produce viable pollen that is chromosomally reduced (Gustaffson 1946-1947, Nogler 1984, Campbell et al. 1991, Van Dijk and Vijverberg 2005). This is particularly the case for apomicts that require pollination to form endosperm (pseudogamy).

Some references on evolution in agamic complexes

Adams, K.L. and J.F. Wendel. 2005. Polyploidy and genome evolution in plants. Curr. Opin. Pl. Biol. 8:135-141.

Bayer, R. J. and G.T. Chandler. 2007. Evolution of polyploid agamic complexes: A case study using the Catipes group of Antennaria, including the A. rosea complex (Asteraceae: Gnaphalieae). Pages 317-336 in Horandl, E., U. Grossniklaus, P.J. van Dijk, and T.F. Sharbel (eds.). Apomixis: Evolution, mechanisms, and perspsectives. A.R.G. Gantner Verlag. Rugell, Lichtenstein.

Campbell, C.S., C.W. Greene and S.E. Bergquist. 1987. Apomixis and sexuality in three species of Amelanchier, shadbush (Rosaceae: Maloideae). Amer. J. Bot. 74:321-328.

Campbell, C.S., C.W. Greene, B.F. Neubauer and J.M. Higgins. 1985. Apomixis in Amelanchier laevis, shadbush (Rosaceae, Maloideae). Amer. J. Bot. 72:1397-1403.

Campbell, C.S., C.W. Greene and T.A. Dickinson. 1991. Reproductive biology in the Maloideae (Rosaceae). Syst. Bot. 16:333-349.

Campbell, C.S. and W.A. Wright. 1996. Apomixis, hybridization, and taxonomic complexity in eastern North American Amelanchier (Rosaceae). Folia Geobot. Phytotax. 31:345-354.

Carman, J. 1997. Asynchronous expression of duplicate genes in angiosperms may cause apomixis, bispory, tetraspory, and polyembryony. Biol. J. Linn. Soc. 61:51-94.

Carman, J. 2007. Do duplicate genes cause apomixis? Pages 63-91 in Horandl, E., U. Grossniklaus, P.J. van Dijk, and T.F. Sharbel (eds.). Apomixis: Evolution, mechanisms, and perspsectives. A.R.G. Gantner Verlag. Rugell, Lichtenstein.

Curtis, M.D. and U. Grossniklaus. 2007. Amphimixis and apomixis: Two sides of the same coin! Pages 37-62 in Horandl, E., U. Grossniklaus, P.J. van Dijk, and T.F. Sharbel (eds.). Apomixis: Evolution, mechanisms, and perspsectives. A.R.G. Gantner Verlag. Rugell, Lichtenstein.

Fehrer, J., A. Krahulcová, F. Krahulec, J. Chrtek, Jr., R. Rosenbaumová, and S. Bräutigam. 2007. Evolutionary aspects of Hieracium subgen. Pilosella. Pages 359-390 in Horandl, E., U. Grossniklaus, P.J. van Dijk, and T.F. Sharbel (eds.). Apomixis: Evolution, mechanisms, and perspsectives. A.R.G. Gantner Verlag. Rugell, Lichtenstein.

Gustaffson, A. 1946-1947. Apomixis in higher plants. I-III. Lunds Univ. Arssk. 42:1-67, 43:69-179, 181-371.

Kirschner, J. and J. Stepánek. 1994. Clonality as a part of the evolution process in Taraxacum. Folia Geobot. Phytotax. 29: 265-275.

Nogler, G.A. 1984. Gametophytic apomixis. Pages 475-518 in B. M. Johri (ed.). Embryology of the Angiosperms. Springer-Verlag, Berlin.

Noyes, R.D. 2007. The evolutionary genetics of apomixis in Erigeron sect. Phalacroloma (Asteraceae). Pages 337-358 in Horandl, E., U. Grossniklaus, P.J. van Dijk, and T.F. Sharbel (eds.). Apomixis: Evolution, mechanisms, and perspsectives. A.R.G. Gantner Verlag. Rugell, Lichtenstein.

Otto, S.P. and J. Whitton. 2000. Polyploid incidence and evolution. Ann. Rev. Genetics 34:401-437.

Richards, A.J. 1986. Plant Breeding Systems. Allen & Unwin. London.

Savidan, Y. 2007. Apomixis in higher plants. Pages 15-22 in Horandl, E., U. Grossniklaus, P.J. van Dijk, and T.F. Sharbel (eds.). Apomixis: Evolution, mechanisms, and perspsectives. A.R.G. Gantner Verlag. Rugell, Lichtenstein.

Soltis, P.S. and D.E. Soltis. 2000. The role of genetic and genomic attributes in the success of polyploids. Proc. Natl. Acad. Sci. USA 97:7051-7057.

Soltis, D.E., P.S. Soltis, and J.A. Tate. 2003. Advances in the study of polyploidy since plant speciation. New Phytologist 161:173-191.

Stebbins, G.L. 1950. Variation and evolution in plants. Columbia University Press. New York.

Stebbins, G.L. and E.B. Babcock. 1939. The effect of polyploidy and apomixis on the evolution of species in Crepis. J. Hered. 30:519-530.

Tate, J. A., D.E. Soltis, and P.S. Soltis. 2005. Polyploidy in plants. Pages 372-426 in T.R. Gregory (ed.). The evolution of the genome. Elsevier Academic Press, Amsterdam.

Van Dijk, P.J. and K. Vijverberg. 2005. The significance of apomixis in the evolution of angiosperms: a reappraisal. Pages 101-116 in F.T. Bakker, L.W. Chatrou, B. Gravendeel, and P.B. Pelser (eds.). Plant species-level systematics: New perspectives on pattern and process. Regnum Vegetabile Vol. 143. A. R. G. Gantner Verlag, Ruggell, Liechtenstein.

Vijverberg, K. and P.J. van Dijk. 2007. Genetic linkage mapping of apomixis loci. Pages 137-158 in Horandl, E., U. Grossniklaus, P.J. van Dijk, and T.F. Sharbel (eds.). Apomixis: Evolution, mechanisms, and perspsectives. A.R.G. Gantner Verlag. Rugell, Lichtenstein.

Weber, J.E. and C.S. Campbell. 1989. Breeding system of a hybrid between a sexual and an apomictic species of Amelanchier, shadbush (Rosaceae, Maloideae). Amer. J. Bot. 73:341-347.

Weber, H.E. 1996. Former and modern taxonomic treament of the apomictic Rubus complex. Folia Geobot. Phytotax. 31:373-380.

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