Low seed viability of a rare aster

 By Handley & Tronstad

 

Desert yellowhead (Yermo xanthocephalus)

Desert yellowhead (Yermo xanthocephalus) is a rare perennial plant, known from two populations in Wyoming, USA. The plant grows up to 30 cm tall with many bright yellow flowerheads that bloom during mid-summer in a sparsely vegetated, semi-arid ecosystem of big sagebrush (Artemisia tridentata), bunchgrass and cushion plants. Information about the reproductive ecology of desert yellowhead is sorely lacking. We measured the number and mass of seeds made by desert yellowhead plants when the only pollen available to flowers was from themselves (self-pollination). We also compared the seeds grown when pollinating insects transported pollen and when we supplied excess pollen from a distant plant (hand-pollinated). Additionally, we used vane traps and pan traps to capture pollinators, and we examined pollen carried by bees. The pollen of desert yellowhead was unique, and we could distinguish it from other aster species in the area. Although desert yellowhead can make seeds from its own pollen, pollen transported by pollinators increased the number of seeds capable of producing new plants. Only 12% of flowers produced seeds that had the potential to produce a new plant in the main population and none were able to do so in the second population, suggesting that something limited seed-set. Flowers that received pollen via insects and flowers that received excess pollen from hand-pollination made about the same number and mass of seeds, showing that pollinators were not responsible for the low number of quality seeds made. Nine types of bees carried pollen from desert yellowhead demonstrating that this plant is a valuable source of pollen and nectar for flower visitors. We recommend continued research to address what is limiting seed production to advance the knowledge and management of this declining plant species.

 

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Pollination Systems in Palms (Arecaceae)

male flowers of Caryota sympetala

By Andrew Henderson

The palm family (Arecaceae) is a family of flowering plants with 182 genera and about 2,460 species. Plants have woody, often large stems and occur predominantly in the tropics. Palms are different from many other plants because they have many, small flowers aggregated into large inflorescences. Pollination has been studied in only about 6% of the species. Nevertheless, this review shows that palms have a diverse array of pollinators. A few species have unusual pollination systems. For example, the Asian genus Eugeissona is pollinated by nocturnal mammals (mostly pentailed shrews), who are attracted to the inflorescences by the abundant, alcoholic nectar that is produced by the flowers. A Central American genus, Calyptrogyne, has flowers which open at night and are pollinated by bats who are attracted by the scent and sweet-tasting petals of the flowers. However, most palms seem to be pollinated either by beetles or by bees/flies. There are two main groups of beetle pollinators, weevils (Curculionidae) and sap beetles (Nitidulidae). These have extremely complicated relationships with the palms and their life cycle takes place entirely on palm inflorescences. Adult beetles are attracted to male-phase flowers by both scent and pollen. They are then attracted to the female-phase flowers, usually by deceit, because the female-phase flowers produce the same scent as the male-phase flowers. Beetles mate on the inflorescences, and the females lay eggs on different parts of the flowers or inflorescences. The larvae then develop on the inflorescences. Bees and flies have a less complicated relationship with palm inflorescences. They are attracted to flowers by sweet scent, pollen, and nectar. One interesting feature of palm inflorescences is that beetle-pollinated species attract bees/flies to their inflorescences, and bee/fly-pollinated species attract beetles. In some cases we can see a ‘switch’ from a beetle-pollinated species to a closely related species with bee/fly pollination. However, it is not clear what causes this switch. One general conclusion of this review is that the interaction between palms and their pollinators is very complicated and we still do not know much about it. 

Read the scientific publication in JPE