Chromosome diversity and evolution in Liliaceae.

Abstract

There is an extensive literature on the diversity of karyotypes found in genera within Liliaceae, but there has been no attempt to analyse these data within a robust phylogenetic framework. In part this has been due to a lack of consensus on which genera comprise Liliaceae and the relationships between them. Recently, however, this changed with the proposal for a relatively broad circumscription of Liliaceae comprising 15 genera and an improved understanding of the evolutionary relationships between them. Thus there is now the opportunity to examine patterns and trends in chromosome evolution across the family as a whole. Based on an extensive literature survey, karyo-morphometric features for 217 species belonging to all genera in Liliaceae sensu the APG (Angiosperm Phylogeny Group) were obtained. Included in the data set were basic chromosome number, ploidy, chromosome total haploid length (THL) and 13 different measures of karyotype asymmetry. In addition, genome size estimates for all species studied were inferred from THLs using a power regression model constructed from the data set. Trends in karyotype evolution were analysed by superimposing the karyological data onto a phylogenetic framework for Liliaceae. Combining the large amount of data enabled mean karyotypes to be produced, highlighting marked differences in karyotype structure between the 15 genera. Further differences were noted when various parameters for analysing karyotype asymmetry were assessed. By examining the effects of increasing genome size on karyotype asymmetry, it was shown that in many but not all (e.g. Fritillaria and all of Tulipeae) species, the additional DNA was added preferentially to the long arms of the shorter chromosomes rather than being distributed across the whole karyotype. This unequal pattern of DNA addition is novel, contrasting with the equal and proportional patterns of DNA increase previously reported. Overall, the large-scale analyses of karyotype features within a well-supported phylogenetic framework enabled the most likely patterns of chromosome evolution in Liliaceae to be reconstructed, highlighting diverse modes of karyotype evolution, even within this comparatively small monocot family.