Watanabe et al. 1999

Watanabe K, Yahara T, Denda T, Kosuge K. 1999. Chromosomal evolution in the genus Brachyscome (Asteraceae, Astereae): Statistical tests regarding correlation between changes in karyotype and habit using phylogenetic information. Journal of Plant Research 112: 145-161.

These authors used most of the described species in a large genus of Australasian flowering shrubs to test hypotheses regarding chromosome evolution and life-history. Brachyscome contains about 80 species; these authors examined 73, plus 8 other species in related genera. The genus includes a wide range of karyotypes, from 2n=4 up to 2n=36, with a mode of 2n=18. The species are distributed across a strong environmental gradient, from high-raingfall coasts and mountains to variable and semi-arid areas in central Australia. Lower chromosome numbers have been previously reported for species occuring in dryer areas, though there is a strong phylogenetic signal associated with those data.

The authors state in the Introduction that the main purpose of this study was to find “some evolutionary trends” in karyotypes and to test the correlations between changes in chromosome number, karyotype, and life-history habit using independent contrasts and a phylogenetic tree. They did this by asking three main questions:
1. Has the reduction in chromosome number occurred in correlation with the evolution of annuality?
2. Has the reduction in total chromosome length occurred in correlation with the reduction in chromosome number?
3. Have karyotypic asymmetry and length heterogeneity increased in correlation with the reduction in chromosome number?

Their phylogenetic tree was based on Denda et al. (in press), who used 47 taxa in this genus and the matK sequence. They resolved polytomies with morphological, anatomical, and cytological data, being careful not to make any assumptions about the directionality of chromosome evolution to “avoid a circular argument”.
Their main finding was that reductions in chromosome number and total chromosome length (i.e. genome size) were indeed associated with a shift from wet habitats to drier and more variable habitats, and strongly associated with a shift from perennial to annual life-history. These data support the existing hypothesis that annual life-history contributes to selection for smaller genomes.

An interesting section in the Discussion maintains that even if smaller genomes are selected for in annuals, it is still necessary to explain why large genomes would be maintained in perennials, in which it would also seem advantageous to have small genomes and rapid growth and early reproduction. The hypothesis proposed here involves population size: perennials (at least in this genus) have much larger effective population sizes than annuals, apparently due to the ephemeral and variable nature of the more xeric habitats typical of annual species. The neutral or mildly advantageous translocations involved in chromosome reduction and loss would not be as likely to become fixed in the larger effective populations of perennial species.

The model of chromosome reduction used in this paper involves translocation of chromosome segments, followed by loss of the centromere on the donor chromosome and associated loss of remaining segments. They found clear evidence supporting a history of large translocations in this genus, which is also consistent with a model of genome dynamics characterized by frequent small increases in genome size by proliferation of transposable elements and larger, rarer losses by translocation and centromere loss.