Section 9 — Population Genetics 



L. perenne inbreds, and in a hybrid with L. peren- 

 ne cytoplasm, which were strongly depressed by 

 high night temperatures. Responses to day 

 temperature depended on the genotype, sup- 

 porting earlier results on such interactions in 

 ryegrass. Long days (16hr) produced more 

 growth in all material than did short days 

 (8 hr). Unnatural conditions, such as very short 

 days (4 hr), cold days and warm nights, or 

 changes in temperature at noon, caused de- 

 pressions. Stress conditions caused by water 

 and nutrient shortage, excessive defoliation, and 

 very low light intensity also depressed growth, 

 and interacted with the genotypes in various 

 measurements. Heterosis and inbreeding de- 

 pression were enhanced by such conditions. 

 Under any one condition, variation within clones 

 was relatively low, depending on plant type but 

 not on heterozygosity, and variation between 

 progenies was generally higher than within 

 progeny variation. The latter conformed to 

 expectations of the homeostasis theory as the 

 mean coefficients of variation over all environ- 

 ments increased linearly with decreasing heter- 

 ozygosity, but it also depended on genotype 

 and environment. Conversely, growth of indi- 

 vidual clones under different environments was 

 in some cases most uniform in the low producing 

 inbred lines. 



'9.4. Ecotypic Variation in Response to Photo - and 

 Thermoperiod of Seedlings of Douglas - fir 

 (Pseudotsuga menziesii (Mirb.) Franco). 



H. Irgens-Moller (Corvallis, U.S.A.). 



The response of young Douglas fir seedlings to 

 long and short photoperiods varies with the 

 geographic origin of the seed. Seedlings from 

 maritime areas show less sensitivity to changes 

 in photoperiod than do seedlings from conti- 

 nental areas. Similarly, the response to differ- 

 ent thermoperiods also varies with seed origin. 

 At a constant temperature of 20 C and 8-hr 

 photoperiods a large percentage of seedlings 

 from Arizona go through 2-3 periods of dor- 

 mancy as opposed to only one in seedlings from 

 Vancouver Island, B. C. Low night temperature 

 (3°C) causes not only depression of growth for 

 all sources tested but an earlier induction of 

 dormancy in the seedlings from continental 

 areas than in those from coastal areas. 



After dormancy is induced and the seedlings 

 are exposed to 16-hr photoperiods and green- 

 house temperatures, growth is resumed consider- 

 ably earlier in the seedlings fiom continental 

 areas than in those from coastal areas. However, 



the seedlings previously exposed to a constant 

 20°C remain dormant for a considerably longer 

 period than those exposed to a low night temper- 

 ature. It is suggested that ecotypic differentiation 

 in long-lived species such as forest trees may 

 profitably be studied by short-term studies of 

 differences in seedling responses to photo- and 

 thermoperiod. 



Supported by grants from National Science 

 Foundation, Washington, D.C. 



To be published in Forest Science, 1964-65. 



9.5. Genetics of Drosophila subobscura Populations 

 in Greece. C. B. Krimbas (Athens, Greece). 



Data concerning the type of inversions and 

 the indexes of free recombination (IFR) of 

 several populations of D. subobscura in Greece 

 are reported. An ecological description of the 

 sites in given. Some of these data are the follow- 

 ing: 



In Mount Parnes a population living in a 

 fir forest (altitude 1100m, rainfall 1000mm) had 

 an IRF of 85-6 ±1-2 and of 85-2 ±1-2 in 

 surveys of two successive recent years. 



In Mount Ainos, Cephallonia, a fir forest the 

 population (alt. 1200m, rainfall 1200mm) had 

 an IFR of 84-3 ±1-2. 



Three populations living in citrus orchards, 

 in Athens (alt: 100m, rainfall 400 mm), in 

 Alikianou, Creta (alt. 100m, rainfall 1040mm) 

 and in Poros (alt. 50m, rainfall 400 mm) had 

 IFR's of 83-8 ±1-5, 850 ±1-2 and 770 ±1-6 

 respectively in 1962. The previous year a small 

 sample of Poros population had an IFR of 

 80.1 ±3.8. 



Attempts to correlate IFR with ecological 

 characteristics of these and other European and 

 Middle East populations are reported. 



Results are also given of experiments, now in 

 progress, concerning the measurement of body 

 weight, wing length, developmental time, 

 dessication resistance, longevity and fertility of 

 the individuals of these populations, their inter- 

 populations Fi's and F2's. 



These data are discussed in relation to 



(a) the Dobzhansky-Carson hypothesis of the 

 pattern distribution of the inversion polymorph- 

 ism within the area of geographical expansion 

 of the species, 



(b) the existence of an integrated gene pool in 

 each population as far as the measured fitness 

 components are concerned and 



(c) the natural selection of the complex group 



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