Evolution and pollination of New Zealand Myosotis (Boraginaceae).
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The indigenous New Zealand species of Myosotis all belong to the southern section Exarrhena. The sister group to the southern species appears to be the discolor-group of eastern Africa and Western Europe. It is postulated that the genus has a long history in New Zealand and is not the result of recent immigration from the northern hemisphere. In New Zealand, extensive speciation has occurred. Approximately 47 species are recognised. Radiation has proceeded in several different directions with respect to floral form. Several floral syndromes are recognised including tube-, funnel- and brush-blossoms. Some species have tended towards autogamy with a reduction in herkogamy, dichogamy, pollen:ovule ratio and petal size. Other species have become highly dependent on particular flower visitors including ants, beetles, flies and moths. All species are protogynous. The pollination of one species, M.colensoi was investigated in detail. This species is self-compatible, but because of strong herkogamy requires a pollinator to deposit pollen on the stigma. It is visited primarily by a tachinid fly, Protohystricia huttoni. An investigation of the rates of pollen removal and deposition confirm that the female function is satisfied more quickly than the male. When conditions are unfavourable for pollination, however, there may be considerable overlap of the two functions. The number of flowers borne simultaneously on a plant of M.colensoi varies greatly. Larger plants receive more visits from P.huttoni and mature more seed. The visitation rate and male and female success per flower differs little between large and small plants, however. P.huttoni often visits many flowers at one bout on big plants suggesting that geitonogamous self-pollinations may be frequent. However, the proportion of flowers visited in one bout declines as the size of the display increases. This, coupled with a high level of pollen carryover, allows a considerable portion of the offspring to be the result of cross-pollination. It is suggested that other plants may be able to relieve some of the effects of geitonogamy by evolving features that increase pollen carryover. P.huttoni exhibits considerable directionality when moving over plants. It is suggested that such movement minimises the risk of encountering previously visited flowers.