Sea sandwort from Surtsey: chromosomal evidence of active evolution via wide-hybridization

Abstract

Sea sandwort (Honckenya peploides) was among the first species of vascular plants colonizing Surtsey. It is a member of the carnation family, Caryophyllaceae, a coastal plant with circumpolar distribution. The species is dioecious comprising separate female and hermaphrodite (male) plants. Our previous study of this plant revealed high molecular polymorphism, indicating rapid expansion and multiple origins, but low genetic differentiation, suggesting gene flow on Surtsey. The maintenance and/or expansion of populations with high gene diversity on the island are most likely fostered by several factors, one of them being the polyploid nature of the study species providing fixed heterozygosity. We therefore investigated chromosome number diversity of H. peploides from Surtsey, in comparison with accessions from Heimaey and other locations within and outside Iceland. Seeds were germinated with and without cold stratification. Chromosomes were isolated from root tips using the cellulase-pectinase enzymatic squash method. DAPIstained chromosomes were counted from microscopic images that were taken at 1000x magnification. The results show that the most common 2n somatic chromosome number of this species is 68 (2n=4x=68), but a tetraploid cytotype with 66 chromosomes also exists. The karyotype analysis shows that the species is an autotetraploid, most likely originating via chromosome doubling (whole genome duplication) in a diploid ancestor. Numerous other 2n numbers were discovered, from the diploid number 2n=2x=34 in Heimaey to several different numbers between 40 and 64. The triploid hybrid numbers 2n=3x=51 (50-52) were discovered from both Surtsey and Heimaey, as well as from other regions. Triploid hybrids serve as a bridge promoting gene flow within populations, promoting heterozygosity in the tetraploid species. All other numbers are aneuploids, most likely deriving from back-crossing of triploid hybrids and the euploid parents. The presence of aneuploids across the species distribution range is due to its ability to propagate asexually by clonal expansion. The presence of the lower ploidy levels within species, together with the extensive aneuploidy, may be an evolutionary characteristic of a pioneering plant, with great dispersal ability and genetic diversity, such as sea sandwort.