COMPONENTS OF PHENOTYPIC SELECTION: POLLEN EXPORT AND FLOWER COROLLA WIDTH IN IPOMOPSIS AGGREGATA

In the hummingbird-pollinated herb Ipomopsis aggregata, selection through male function during pollination favors wide corolla tubes. We explored the mechanisms behind this selection, using phenotypic selection analysis to compare effects of corolla width on two components of male pollination success, pollinator visit rate and pollen exported per visit. During single visits by captive hummingbirds, flowers with wider corollas exported more pollen, and more dye used as a pollen analogue, to stigmas of recipient flowers. Corolla width was less strongly related to visit rate in the field, and had no direct effect on visit rate after nectar production and corolla length were controlled for. Moreover, the phenotypic selection differential was 80% higher for the effect on pollen exported per visit, suggesting that this is the more important mechanism of selection.     

Intercolonial differences in raiding activities in the Japanese slave-making antPolyergus samurai

Summary Intercolonial differences in raiding activity were investigated in the field on 9 colonies of the slavemaking antPolyergus samurai. Duration of raiding season, the mean start time of the first raiding trip in a day, the mean speed of outbound and inbound trips, and the mean distance to target nests varied significantly among colonies. The variance in start time of raiding was correlated with that in soil temperature and diurnal change pattern in soil temperature at the nest sites. The speed of trips and the first day with trips in the season also correlated with soil temperature at the nest site. Simple environmental factors well explained the observed variances in raiding activity among colonies. Therefore, raiding behaviour ofP. samurai seemed to be a stereotyped behaviour that is regulated through simple environmental factors.     

Stamens are not essential as an attractant for pollinators in females of cryptically dioecious Thalictrum pubescens Pursch. (Ranunculaceae)

 It has been hypothesized that females of some dioecious species maintain stamens that produce sterile pollen as a means of attracting pollinators to promote greater seed set. However, this hypothesis has rarely been tested. This paper examines the role of pollinators in the maintenance of sterile stamens in the cryptically dioecious species, Thalictrum pubescens. Wind was found to contribute to pollination; branches of female T. pubescens enclosed in cheesecloth to exclude insects but not wind were still able to set seed. Therefore, females may not need the stamens for pollination. In 1994, insects were found to discriminate against emasculated female flowers, but this discrimination did not lead to a significant difference in either the amount of pollen received or seed set. In 1995, emasculation was combined with the addition of pollen to determine if emasculated females were pollen limited. No significant effect of pollen addition or emasculation was found. Emasculated branches in the pollen-addition treatment appeared to have slightly lower seed set than non-emasculated branches that received pollen, indicating that the slight reduction in seed set was caused by emasculation rather than pollen limitation. Since stamens do not appear to be maintained as pollinator attractants, other explanations, such as genetic constraints, must be investigated. Received: 4 February 1997 / Revision accepted: 20 May 1997     

Variable thermal sensitivity as output of a circadian clock controlling the bimodal rhythm of temperature choice in the ant Camponotus mus

Along a thermal gradient and under a LD 1212 h cycle, nurse workers of the ant Camponotus mus select for the brood two different temperatures daily: 30.8°C at the middle of the light period (circadian phase = 90°), and 27.5°C 8 h later, during the dark period (CP = 210°). Brood-carrying activity proved to be self-sustained, running its two daily bursts free with a similar period of 23.5 h, under both LL and DD. The LD alternation acted as a strong Zeitgeber. A phase-delay of the LD 1212 h cycle reset the overt rhythm at once, being both daily events locked-on to the delayed light: dark transition. However, changes in expression, non-occurrence, or even splitting of the two daily brood-carrying events during resetting depended on the phase of the delayed DL transition. By comparing the occurrence of activity with predictions based on a threshold curve of thermal sensitivity, results indicated that an immediate resetting of the involved pacemaker actually takes place. Nurse workers do not directly control the total time spent by the brood at the selected temperature. Instead, the endogenously-driven thermal sensitivity triggers their thermal-searching behavior at two critical times of the day, when environmental temperature is expected to reach its maximum and minimum.     

The pollination ecology ofBletilla striata (Orchidaceae)

Anthecological observations of an orchidBletilla striata were carried out in Kobe, Japan. The purplish pink flowers were visited by 26 species of insects belonging to Hymenoptera, Diptera and Lepidoptera, although the flower did not offer food rewards of any kind. Candidates for pollination were seven species of aculeate Hymenoptera with a body size that well fitted the space in between the column and labellum. The most effective candidate was the male of the longhorn beeTetralonia nipponensis, judging from its abundance and legitimate intrafloral behavior. The female ofT. nipponensis and the other six species also contributed to promote out-crossing, but to a lesser extent than the maleT. nipponensis, since they occasionally exhibited illegitimate intrafloral behaviors for pollination and also, their flower visitation rate was lower.     

Genetic architecture of yield and components of yield in mustard (Brassica juncea (L.) Czern & Coss.)

Summary The relative ability of cross- and self-pollen to achieve fertilisation in Brassica was studied by making double pollinations using cross-pollen carrying a dominant seedling marker gene. With simultaneous self- and crosspollination 12–40% self-seed was set, but when cross-pollen was applied to the stigma four hours before self-pollen, only 2–4% self-seed was obtained. In two plants to which cross-pollen was applied at various time intervals after self-pollen there was a tendency for the percentage of self-seed to increase as the time interval increased. In a third plant this trend was not apparent, probably because of a greater degree of self-incompatibility. The competitive advantage of the first pollen to arrive on the stigma is discussed in relation to the strength of the self-incompatibility and the sib problem in F₁ hybrid brassicas.