Te Papa Botany researchers study genome size in hebes

Te Papa Botany researchers Heidi Meudt, Jessie Prebble and Phil Garnock-Jones have recently co-authored a new paper in the Botanical Journal of the Linnean Society on the genus Veronica, which includes northern speedwells and New Zealand and Australian hebes. This paper is the first major publication from Heidi’s research stay in Oldenburg, Germany as an Alexander von Humboldt Experienced Research Fellow.

There are approximately 450 species in the genus Veronica, about one-third of which are endemic to the Southern Hemisphere. In New Zealand, Veronica is the plant genus with the most native species (ca. 120 species). Plants from three-quarters of all Veronica species–including all Southern Hemisphere species–are polyploids. This means they have multiple (4, 6, 8, 10 or 12) sets of chromosomes, unlike people and most animals, which usually have only 2 sets of chromosomes.

One of the species of New Zealand hebes, Veronica lycopodioides, which was included in our study. The plants are hexaploid, which means they have 6 sets of chromosomes. Hand-coloured lithograph by John Nugent Fitch, 1894, England, purchased 2004.

More sets of chromosomes usually equates to more DNA. Since 1967, we’ve known how many chromosomes are found in the cells of most of the New Zealand hebes, but we haven’t known how much DNA they hold. The amount of DNA that all sets of chromosomes contain is called “genome size”. Prior to this study, the genome sizes of 40 species of Veronica were known, but only one of those was from the Southern Hemisphere.

For the new study, we measured genome size in 174 individuals representing 102 species, including 54 species from New Zealand and Australia. We now know that all hebes and speedwells have relatively small genome sizes (0.26–3.19 picograms) compared to other plants and animals, roughly comparable to that of rice in this figure.

In general, Southern Hemisphere Veronica plants have more DNA (larger genomes) than most of the Northern Hemisphere species. But when we compare the amount of DNA in one chromosome set only, Southern Hemisphere Veronica plants tended to have less DNA (smaller genomes).

This means DNA loss, or “genome downsizing” has occurred in Veronica, as it has in other polyploid plant groups. We think that such “genome downsizing” may have something to do with why there are so many species of Veronica in New Zealand, but testing these ideas will require more studies.

One of the Northern Hemisphere species included in our study, water speedwell or Veronica anagallis-aquatica, that is also naturalised in New Zealand, SP089473/A. The plants are tetraploid, with 4 sets of chromosomes. Photo by Leon Perrie © Te Papa.

In addition to studying genome size in Veronica, we also estimated several dates of interest in its evolutionary tree using molecular dating methods and DNA sequence data. For Veronica, most of its milestones happened in the Miocene (23-5 million years ago, Mya), including:

  • 25–15 Mya: Origin of the genus Veronica.
  • 14–17 Mya: Diversificaiton of Northern Hemisphere lineages.
  • 15–8 Mya: Colonisation of the Southern Hemisphere.
  • 10–5 Mya: Start of New Zealand diversification, which intensified during the Pliocene and Pleistocene (<5 Mya).

Collaboration with Dirk Albach’s lab is still ongoing, with more research and publications on the evolution of Northern and Southern Hemisphere Veronica in the pipeline. Thanks to the Alexander von Humboldt Foundation for funding.

Heidi M. Meudt, Blanca Rojas-Andrés, Jessica M. Prebble, Evonne Low, Phil J. Garnock-Jones, and Dirk C. Albach. 2015. Is genome downsizing associated with diversification in polyploid lineages of Veronica? Botanical Journal of the Linnean Society 178: 243–266. doi:10.1111/boj.12276
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