× Pucciphippsia vacillans

× Pucciphippsia vacillans
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Monocots
Clade:	Commelinids
Order:	Poales
Family:	Poaceae
Subfamily:	Pooideae
Genus:	× Pucciphippsia
Species:	× P. vacillans
Binomial name
× Pucciphippsia vacillans (Th.Fr.) Tzvelev

× Pucciphippsia vacillans is a member of the nothogenus × Pucciphippsia, formed by the hybridization of Phippsia and Puccinellia.^[1]

× Pucciphippsia vacillans is a grass (Poaceae). The plant is perennial, growing between 15 and 23 centimeters in height.^[2] × P. vacillans is caespitose, meaning it grows densely at the base, forming a thick mat. The plant has fibrous roots and sheathing leaves which grow alternately along the stem. The leaves are glabrous. Sheath collars and ligules are present. Each leaf blade is 30 to 80 mm long. × P. vacillans is a monocot, and therefore the leaves have parallel venation. There is a distinct mid-vein running down the center of each leaf blade.

Each plant has two or more flowering stems and panicle inflorescences.^[3] The panicles are usually tinged purple. Each panicle is composed of spikelets, and the spikelets each contain one or more floret. The florets are bilaterally symmetrical and bisexual.^[2] Like many grass species, the florets are not showy. The perianth is formed by lodicules (a pair of small, scale like, modified petals which help to push open the flower).^[4] Each flower has a superior compound ovary, formed from three fused carpels, and each carpel contains a single ovule.^[2] The flowers also contain three anthers, however these anthers are shriveled and non-functional.^[3] Because of this, the ovary never matures into a fruit, making P. vacillans sterile and only produced as a product of hybridization.

× Pucciphippsia vacillans is easily confused with Phippsia algida, but × P. vacillans has longer leaf blades (Phippsia algida has leaf blades from 8 to 20 mm), more than one floret per spiklet, and panicles with ascending, rather than erect, branches.^[5]

× Pucciphippsia vacillans is native to arctic regions of the world. Relatively limited observations of it may be due to its scarcity or to its obscurity and the fact that in grows in parts of the world where there is very little human inhabitation. It was thought to be limited to Svalbard (Norway) and Novaya Zemlya (Russia),^[3] until it was observed on a variety of islands in Nunavut (Canada)^[2] and in Greenland.^[1]

× Pucciphippsia vacillans lives in some of the harshest and least inhabited northern regions. It grows in wet meadows, around the edges of ponds, and in seepage zones.^[2] It generally grows well in areas that are permanently or frequently wet, often in fine grained gravel or moist to wet thick moss carpets.^[3]

Hybridization is relatively common among grasses and often associated with polyploidy.^[6] Genetic analysis has shown × P. vacillans to be triploid. Additionally, no mature fruits produced by × P. vacillans have been observed, the anthers have not been observed to contain any pollen, and the ovaries are frequently atrophied.^[7] That × P. vacillans is likely sterile provides strong support for the hypothesis that it is a hybrid species. Since it was first recognized in 1869, the identity of the parent species of × P. vacillans have been disputed.^[7] × P. vacillans is triploid, and it is likely that is the product of the hybridization of a diploid species and tetraploid species. Genetic analysis has shown × P. vacillans to be very closely related to Phippsia algida (tetraploid), Phippsia concinna (tetraploid), and Puccinellia vahliana (diploid), meaning that Puccinellia vahliana is likely one parent. Statistical analysis of morphology has shown that × P. vacillans is an intermediate between Phippsia algida and Puccinellia vahliana,^[5][7] making Phippsia algida the most likely second parent, rather than Phippsia concinna. Due to their similar morphology, habitat, and reproduction and dispersal methods, there is dispute as to whether P. concinna and P. algida are different species at all, however their distinct phylogeographies support their recognition as distinct species.^[8]

Although the Arctic is considered to have low plant species diversity, arctic flora show high diversity within species and among populations.^[9] Reproductive isolation of populations is frequent. There are few pollinators in the arctic and glaciation fragments ranges, isolating populating in disjoint refugia. Subject to strong genetic drift, within less than 500 generations these isolated populations can develop into their own incipient species, incapable of producing viable offspring with plants from other population.^[10] There is thought to be high biological species diversity within taxonomically recognized species of arctic flora. Hybridization is also common among arctic flora and often results in polyploidy.^[9] Frequent reproductive isolation and hybridization makes species delineations difficult to define. Additionally, the subfamily of which × P. vacillans is a member of (Pooideae) has been shown to have deep phylogenetic incongruence.^[6] The case of P. vacillans is not necessarily unusual, and it is somewhat representative of the difficulties in understanding phylogenies and species delineations among arctic flora and the Pooideae subfamily.