Draba verna the spring draba, shadflower, nailwort, common whitlowgrass, vernal whitlow grass, early witlow grass or whitlow-grass is a species of plant in the mustard family, Brassicaceae. D. verna has the unique trait of bifid petals, not found anywhere else in the genus Draba. The plant consists of a few flowers with branching stems and the leaves are focused around the base of the plant. The seeds are located in the flower but are not equipped with any sort of wind dispersal adaptation.
Distribution
The native range of Draba verna includes parts of Great Britain and Central Europe, specifically parts of Denmark, The Netherlands, and Belgium and Hungary. It is postulated that Draba verna was brought to North America when the European colonizers came to the New World. Draba verna originated in Europe and naturalized in the United States, where it has been found mostly east of the Mississippi River and west of the Rocky Mountains. Draba verna has been found in the high Semien Mountains of Ethiopia as well as in the Bale Mountains, which is a unique location for this species to grow since it is reasonably further south and in a more tropical region than the Draba verna growing in the United States. It is found in Chile and Patagonia, Argentina as well and is predicted to have been brought from Europe by colonizers. It was first recorded in 1901. It has also been found in Canada, specifically recorded in Nova Scotia, New Brunswick, British Columbia, Ontario, and Quebec. Draba verna in this region is presumed to have been dispersed from Europe partially by campers and hikers since this species is typically considered a weed found in camping grounds or open fields. Regions of Great Britain and surrounding islands, most of the mainland of Europe, as displayed in the image on the right, are other places D. verna has been found. In the continent of Asia, D. verna has been recorded in Kazakhstan, Uzbekistan, and Kyrgyzstan.
Phenology
In the northeastern United States, Draba verna typically flowers in early spring, March–May. The seeds fall to the soil and remain there until germinationin September or October. Germination doesn't occur in the summer months because the seeds are dormant and they need about 3 months to after-ripen before they can germinate successfully. Draba verna typically reproduces by self-pollination or selfing in mid- to late April when the buds begin to flower and will disperse when the seeds are mature.
Germination
Humidity
In experiments run on Draba verna seeds, trends that are important to the germination of the seeds have been noted. In the range of 0% to 20% relative humidity, the seeds of Draba verna did not after-ripen, a term used to show maturing of the seeds before germination. However, seeds at this humidity did remain viable. At a higher humidity of 30% to 40%, there was a small percentage of seeds that after-ripened. At 50% to 60% relative humidity, there was a high percentage of seeds of the Draba verna that after-ripened into mature seeds. From 70% to 100% humidity, the seeds rotted from too much exposure to a moist environment.
Cold
The cold affects these plants since they are winter annual plants. When they are exposed to colder temperatures a set of signals is released within the plant in order to survive the frost. In temperate climates such as those where D. verna is found the plants cease their growth in the autumn when the cooler temperatures arrive and the sunlight is less available. This prepares the plant for seasonal acclimation to the cooler temperatures. When these plants don't get the chance to acclimate to the cold over an “autumnal season” then their mortality rate drastically increases.
Light
It has been found that these seeds germinate best when they receive sufficient amounts of water from the surrounding soil, and receive light until they are about 5–7 weeks old, then being buried or covered under the soil, allowing them to grow and mature. If the seeds are covered before receiving 5–7 weeks of sunlight, they will not germinate well in the autumn. Of all the factors affecting the germination of Draba verna, light is the lowest influencer.
Phylogeny
Studies performed have tried to map the evolutionary history of the genus Draba, with D. verna included. Due to certain unique traits that D. verna possess it is believed that this species is a very ancestral species in the genus Draba, referred to as one of the Core Draba. It has been found to have a very highly variable number of chromosomes, 2n=14, 28, 30, 32, 36, 38, 39, 52, 58 to 64 pairs of chromosomes. The base chromosome number has not been determined yet, giving proof that this species has a complex evolutionary past in order to become as widespread as it is. It is the only species in the genus Draba that is naturalized worldwide. It is hypothesized that one of the causes of the variable polyploidy in this genus us due to the Pliocene and Pleistocene glaciation cycles, which were the dramatic climate changing events over 40 thousand years ago.
Ploidy
D. verna possesses the characteristic of polyploidy, having more than two sets of chromosomes. When there is a meiotic or mitotic error, and there are multiple copies of the genome. Polyploids form at reasonably high frequency in flowering plants, suggesting that plants have a remarkably high tendency for polyploidy. There are several advantages to polyploidy, such as, observation of hybrid vigor, or heterosis, which is the polyploid offspring of two diploid parents is more vigorous and healthy than either of the two diploid parents. Polyploidy is a class of mutation that results in a mitotic doubling and failure in cell division. It appears that this is very common in plants, in the method of unreduced pollen. Since perennial plants are able to undergo selfing or asexuality when they are unable to find a mate, it appears polyploidy is more common in the perennial species of plants for that reason. The genus as a whole is vastly widespread and has a wide range of ploidy located in almost every continent. It has been researched on the relationship between the types of ploidy being specific to a certain continent, hinting that there is some sort of environmental pressure encouraging polyploidy to occur. Specifically, in the Draba verna species, they are referred to the aneuploid type in the genus, for the abnormal number of chromosomes. The aneuploid D. verna is typically found in mountainous regions. The suspected cause of the high variation of ploidy in these mountainous regions is due to the pressures of the changing environment. When the species is facing more stress from the environment in there is a selective pressure for apomixis, or asexual reproduction, which may give an answer to this plant's tendency to have an uneven ploidy number. On average, in the Draba genus the higher the elevation and latitude of the species the more ploidy the species possesses.
