Chromosomal fission accounts for small-scale radiations in Zamia (Zamiaceae; Cycadales)

Abstract

Zamia is unique among Cycadales in its diversity of morphology, ecology and chromosome numbers. The chromosome numbers in Zamia range from 16 to 28, excluding 20, manifest as both interspecific and intraspecific series. It has long been recognized that Robertsonian transformations (chromosomal fission or fusion) probably dominate karyotype evolution in Zamiaceae, although it has been debated whether chromosome numbers are increasing or decreasing. We re-analyse published karyotypes of Zamia spp., relating both chromosome forms and sizes to recent phylogenetic data. We show that karyotype evolution is most probably moving towards increased asymmetry, with higher numbers of smaller chromosomes, thus supporting chromosomal fission. We also address additional hypotheses for increasing chromosome numbers, namely pericentric inversions and unequal translocations. Finally, we discuss the role of these chromosomal changes in evolutionary radiations.

Karyotype evolution in apomictic Boechera and the origin of the aberrant chromosomes

“…However, the common acro‐ and telocentric chromosomes that prevail in the karyotypes of several plant groups, were traditionally interpreted as having resulted from chromosome fission events (see Jones, ). Indeed, when karyomorphological data are plotted on phylogenetic trees, fission events seem to occur, for example, during genome evolution in cycads (Olson and Gorelick, ), Crocus (Brighton, ), the slipper orchids (Cox et al ., ), Lycoris (Shi et al ., ) or in the classical example of Campanula persicifolia (Darlington and La Cour, ). Whereas these fission events in plants were deduced only from chromosome numbers, chromosome morphology and/or meiotic pairing configurations (Jones, ), data on the molecular mechanism of centric fissions as well as on the detailed genomic composition of metacentric chromosomes and their telocentric derivatives are scarce.…”

Dancing together and separate again: gymnosperms exhibit frequent changes of fundamental 5S and 35S rRNA gene (rDNA) organisation

“…It will be interesting to determine chromosomal position of rDNA in other species in order to see whether the (sub)telomeric locations are favoured by the L-type arrays. It is also an open question of whether inversions, fusions (Olson and Gorelick, 2011) and other fundamental changes in chromosome structure (Leitch and Leitch, 2012) that accompany the long (300 Myrs) history of gymnosperm evolution contribute to switches in rDNA arrangements.…”

Comparative linkage maps suggest that fission, not polyploidy, underlies near-doubling of chromosome number within monkeyflowers (Mimulus; Phrymaceae)

“…Such centromere repositioning events, accompanied by a shift in the centromere-associated tandem DNA repeat, appear to have been important in chromosome number evolution within the cucurbits (Han et al, 2009). Nonetheless, polyploidy has been concluded to be the major source of chromosome number increases in plants (Jones, 1998), except in a few odd lineages such as Zamia (Olson and Gorelick, 2011), slipper orchids (Cox et al, 1998) and sedges (Chung et al, 2012). In sedges, which have holocentric chromosomes, fission may be common in part because the lack of localized centromeres allows breakage anywhere along a chromosome with relatively low risks of meiotic dysfunction (Chung et al, 2012).…”