Karyotype and Genome Evolution in Pteridophytes
Although genomics is a field often discussed as a recent development brought upon by the advent of next-generation sequencing technologies, the roots of genomics extend to at least the early twentieth century. Rather than running a gel or assembling seque
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Michael S. Barker
Contents 15.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
15.2
High Chromosome Numbers and Hypotheses . . . . . . . . . . 246
15.3
Recent Advances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
15.4
Future Directions for Fern Nuclear Genomics . . . . . . . . . 251
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
M.S. Barker (*) Department of Ecology & Evolutionary Biology, University of Arizona, Tucson, USA e-mail: [email protected] I.J. Leitch et al. (eds.), Plant Genome Diversity Volume 2, DOI 10.1007/978-3-7091-1160-4_15, # Springer-Verlag Wien 2013
15.1
Introduction
Although genomics is a field often discussed as a recent development brought upon by the advent of next-generation sequencing technologies, the roots of genomics extend to at least the early twentieth century. Rather than running a gel or assembling sequence reads on a server, early cytologists squashed and stained actively dividing cells and viewed them under a microscope to reveal a variety of chromosome features including numbers, sizes, and pairing behaviour. These data revolutionized our perspective of plant species, and provoked numerous questions about genome evolution, some of which endure today. Among these long-standing questions is how the high chromosome numbers of homosporous ferns and lycophytes evolved and are maintained. By the 1950s, it was clear that fern nuclear genomes, particularly those of homosporous species, possessed exceptionally high chromosome numbers (Manton 1950). Comparison of chromosome counts among related species of many genera clearly showed that series of currently recognizable polyploids (neopolyploids) such as tetraploids and hexaploids are frequent among ferns and lycophytes, eventually reaching the highest known chromosome count among extant eukaryotes in Ophioglossum reticulatum (n > 600; Khandelwal 1990). These observations indicated that whole genome duplication is an ongoing process and a significant feature of fern and lycophyte evolution. In addition to the abundant occurrence of neopolyploids, it became clear that, with rare exceptions, even the lowest chromosome numbers in each genus were much higher than those of other plant groups. Cytologists studying angiosperms and other heterosporous species rarely required more than fingers and toes to count chromosomes; flowering plants have an average of n ¼ 15.99 chromosomes (Klekowski and Baker 1966). However, homosporous ferns and lycophytes required more digits and patience to count their chromosomes. On average, homosporous fern and lycophyte genomes contain n ¼ 57.05 chromosomes, over three-fold
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more than the average flowering plant (Klekowski and Baker 1966). This striking difference between homosporous and heterosporous plants spawned a number of hypotheses.
15.2
High Chromosome Numbers and Hyp
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