K. Landa

COMPETITIVE EFFECT AND RESPONSE: HIERARCHIES AND CORRELATED TRAITS IN THE EARLY STAGES OF COMPETITION

SUMMARY (1) Competitive ability can be compared between species in two ways: effect of different neighbour species on performance of a single target species or response of different target species to a single neighbour species. In a 5-week glasshouse experiment, an additive design was used for all combinations of seven species as both target and neighbour species to determine if there were consistent hierarchies in competitive effect and/or response, what traits of individuals determined position in these hierarchies, and whether or not effect and response competitive ability were related during the early stages of competition. (2) Five weeks after sowing, significant non-linear regressions of target biomass on neighbour density were found for 59% of the forty-nine species combinations and significant linear regressions on neighbour biomass were found for 51% of the species combinations. The slopes of these regressions represent per-plant and per-gram competition coefficients, respectively. (3) Neighbour species differed strongly in competitive effect per plant. Differences in effect per gram, response per plant, and response per gram were much weaker. Nevertheless, consistent competitive hierarchies were found for both effect and response on both a per-plant and per-gram basis. (4) Different traits determined position in the effect and response hierarchies. Neighbour species with larger seed mass and larger maximum potential mass had stronger per-plant competitive effects, whilst neighbour species with higher maximum relative growth rates had stronger per-gram competitive effects. The reverse of this latter pattern was seen for competitive response: target species with lower maximum relative growth rates were better response competitors. Mean effect and response

Cholinergic Neuronotrophic Factors: Fractionation Properties of an Extract from Selected Chick Embryonic Eye Tissues

Abstract: An aqueous extract derived from selected intraocular tissues of 15‐day chick embryos contains a soluble macromolecular agent which is capable of ensuring the survival of 8‐day chick embryonic ciliary ganglionic neurons in monolayer culture. When this ciliary neuronotrophic factor (CNTF) was concentrated using ultrafiltration and subjected to Sephadex G100 and G200 chromatography, activity was detected in most of the eluted fractions. A peak of the most active fractions was eluted in a region corresponding to a molecular weight of 35–40 ± 103 and contained about 20‐30% of the applied protein. CNTF activity bound readily to DE‐52 cellulose resin at neutral pH and was eluted with NaCl in a narrow region containing about 20‐40% of the applied protein. Gel electrophoretic staining profiles of the active DE52 fraction indicated considerable (but still only partial) simplification in protein composition. While significant CNTF activity losses were incurred in response to each of the above treatments, an active material could be conveniently generated in one working day in milligram amounts having a specific activity of 60,000 trophic units/mg protein. This trophic activity is in the same range as that of the only other known neuronotrophic factor, Nerve Growth Factor.

Cholinergic neuronotrophic factors: III. Developmental increase of trophic activity for chick embryo ciliary ganglion neurons in their intraocular target tissues

Abstract Between stages 34 and 40 in the chick embryo, the ciliary ganglion (CG) undergoes a 50% loss of neurons. Such neuronal death is a common feature in neural development and it has been proposed that neurons are dependent for survival on trophic support from their target tissues. Using an in vitro bioassay it was previously shown in this laboratory that trophic activity for CG neurons is highly concentrated in eye structures containing CG target tissues. In the present study we have found that trophic activity in the eye increases markedly between stages 37 and 39, the time when neuronal death in the ciliary ganglion is ending. Thus, a developmental increase in trophic activity within the eye may be involved in determining neuronal survival in the CG. Furthermore, this study provides the first indication that the trophic content of target tissue is itself developmentally regulated.

SEASONAL DECLINES IN OFFSPRING FITNESS AND SELECTION FOR EARLY REPRODUCTION IN NYMPH-OVERWINTERING GRASSHOPPERS

In this study, I examine the effects of natural and experimentally induced variation in life cycle timing on offspring fitness in Arphia sulphurea and Chortophaga viridifasciata, to understand the selective pressures shaping phenology in these two species of nymph‐overwintering grasshoppers. Because these species lack embryonic diapause, hatching varies over a two month range under natural conditions. I used a cold treatment to delay hatching of some egg pods and extend the natural range of hatching dates. Due to the shorter time for growth and poorer growing conditions late in the fall, late‐hatching nymphs of both species grew to a smaller size before winter and suffered higher overwinter mortality, compared to early nymphs. In addition, late nymphs that did survive the winter became reproductive later in the following years breeding season. Size‐ dependent mortality of offspring during the winter is a strong selective pressure favoring early reproduction in these species. Female adult life history traits appear responsive to the seasonal declines in offspring fitness, in that late‐maturing females began reproducing sooner after adult maturation and reproduced at a more rapid rate, even at the expense of having shorter adult longevity and producing fewer total egg pods. Experimental manipulations were crucial in understanding the fitness consequences of intrapopulation variation in the timing of specific life‐cycle events for these species.

ADAPTIVE SEASONAL VARIATION IN GRASSHOPPER OFFSPRING SIZE

Offspring size has been reported to vary seasonally in a diverse group of organisms: for example, flowering plants, isopods, cladocerans, insects, fish, amphibians, and reptiles (Wellington, 1965; Leonard, 1970; Kerfoot, 1974; Harvey, 1977; Ware, 1977; Howard, 1978; Richards and Myers, 1980; Nussbaum, 1981; Ferguson et al., 1982; Brody and Lawlor, 1984; Cavers and Steel, 1984; Marsh, 1984; Wiklund and Karlsson, 1984; Perrin, 1988; DeMarco, 1989; McGinley, 1989; Dangerfield and Telford, 1990). Seasonal increases and decreases in offspring size are both common, and some populations show even more complex responses. These studies have shown population level changes in offspring size, as well as phenotypic plasticity by individual females, with varying offspring sizes produced among a females successive clutches. Although there has been some discussion relating observed seasonal changes in offspring size with seasonal changes in biotic and abiotic conditions (e.g., Kerfoot, 1974; Brody and Lawlor, 1984; Perrin, 1988), more work is needed concerning whether these seasonal responses are adaptive or not and, if adaptive, what selective pressures favor such plasticity. Many theoretical models have been developed, beginning with Smith and Fretwell (1974), which predict optimal offspring size, based on the relationship between offspring size and offspring fitness (e.g., Smith and Fretwell, 1974; Brockelman, 1975; Pianka, 1976; Parker and Begon, 1986; Winkler and Wallin, 1987; McGinley et al., 1987). Investing fewer resources per offspring is assumed to allow parents to produce more offspring, increasing a parents potential fecundity. However, fewer resources per offspring may decrease offspring fitness, with the result that each individual offspring contributes less to parental fitness. The optimal offspring size maximizes parental fitness: the product of offspring number and offspring fitness. One general prediction of these models is that offspring size should increase under conditions that decrease offspring survival and/or future reproduction (Sibly and Calow, 1983; Taylor and Williams, 1984; McGinley et al., 1987). These models have been explicitly applied to seasonal changes in offspring size (McGinley et al., 1987). Offspring fitness may change seasonally due to changing abiotic and biotic conditions (Dixon, 1976; Lacey, 1982; Ohgushi, 1986; Kalisz, 1986). Consistent withinyear variation in environmental conditions affecting