Hereditus 104: 231-243 (1986) The latitudinal and yearly variation in the timing of micro- sporogenesis in Alnus, Betula and Corylus ALP0 LUOMAJOKI The Finnish Forest Research Institute, Department of Forest Genetics, Helsinki, Finland LUOMAJOKI, A. 1986. The latitudinal and yearly variation in the timing of microsporogenesis in Alms, Re- tula and Corylus. - Hereditas 104: 231-243. Lund, Sweden. ISSN 0018-0661. Received May 10,1985 The timing of the tetrad phase of inicrosporogenesis in five Betulaceae species growing in natural condition5 in Finland was studied by calendar days and by period unit and degree-day heat sums. Latitudinal variation and variation between successive years were examined. To account for the variations found, models using heat sums. critical daylength and joint effects of daylength and temperature sums were tested. The total lengths of meioses and tetrad phases were measured in four tree species. Variation in the phenology of tree species and the underlying different timing systems are discussed with reference to the annual cycle of the generative development in trees. The tetrad phase of the microsporogenesis was usually reached in August in all the species studied. There were no significant differences between the species studied, but latitudinal differences were significant. In contrast to previously studied species (the meioses of which occur in the spring), the meiosis on site occurred carlier the higher the latitude. Within Finland a latitudinal range of about 10 days was observed. Moderate variation between years and high dispersion within trees and populations were also characteristic of Be- tulaceae. Neither a simple temperature-sum or critical-day timing model nor a model on the joint effect of daylight and the temperature could account for all the variation found. Duration of daylight is found to be the most important factor in the timing of Betulaceae meioses. Temperature is considered to be a modifying factor here in contrast to meioses in previously studied species in which the temperature was the main factor. This difference in timing systems is found to exist in adaptative strategies between different parts of the annual cycle rather than between tree species or genera. Alpo Luomajoki, The Finnish Forest Research Institute, Department of Forest Genetics, Unioninkatu 40 A, SF-001 70 Helsinki 17, Finland (present address) In the northern temperature climate there are large 1969). Some conifer meioses also begin in the au- differences in the timing of meioses between various tumn but the meioses are completed no earlier than tree genera. The most essential feature of the timing the next spring. The PMCs have thus to overwinter is the existence of different timing patterns. In a re- dormant in pachytene or diplotene. This type of view by ANDERSONet al. (1969) most of the early lit- timing was recently studied in Larix (ERIKSSONet al. erature on conifer meioses can be found. The con- 1970b; OWENSand MOLDER1971, 1979b; SARVAS ception of the timing of Pseudotsugu meiosis has 1972; HALLand BROWN1976; HALL1979; LUOMAJOKI since changed, and more detailed information of the 1982, 1984), in Pseudotsuga (OWENSand MOLDER other species is now available. 1971), in Thuju (OWENSand MOLDER1971; SIMAKet Most conifer meioses take place in the spring. Re- al. 1974), and in Tsugu (OWENSand MOLDER1971, cent literature is available of Abies (LUOMAJOKI1975). 1982, 1984), of Pica (ERIKSSON~~al. 1970a; SARVAS Following the suit of most conifers, the meioses in 1972; MOIRand Fox 1976; OWENSand MOLDER many deciduous species also occur in the spring. 1979a, 1980; ROZHDESTVENSKII1981; SINGHand Populus has been observed by POSPISIL(1966), EK- OWENS1981; LUOMAJOKI1982, 1984), of Pinus (EK- BERG et al. (1967) and LUOMAJOKI(1984), Quercus by Btmet al. 1972; Ho and OWENS1974; OWENSet al. TucoviC and JOVANOVIC(1970) and Urnus by 1981; LUOMAJOKI1982, 1984) and of Pseudolarix DAHLGREN(1915) and REDENBAUGHet al. (1980). (MFKGEN1977). In several deciduous genera the meioses occur Meiosis in a few species both begins and is com- after mid-summer, i.e., in July or August, or even pleted during the autumn (see ANDERSSONet al. later: This has been observed in Betula by WOOD- 232 A. LUOMAJOKI Hereditns 104 (1986) WORTH (1929), SOKENSEN (1941) and LUOMAJOKIature an equally dominant factor for the timing of (1977,1982) and in Ahus and Corylus by LUOMAJOKImeioses in Alnus, Betulu, Corylus as it was for the (1977, 1982). meioses that occur in the spring (e.g., Abies, Picea There have been few attempts to find a quantita- and Pinus)? tive basis to the timing of tree meioses. One of the The lengths of meioses in the species under study reasons for this was the lack of a suitable heat-sum had also to be appraised, because there is probably system for simulation of development according to no information of these available. This is also the temperatures. Such simulation was much facilitated situation concerning the length of the tctrad stage. after SAKVAS(1972) created his period unit system. Knowing both variables mentioned, the relative du- The definition of the "period unit" is presented in ration of thc tetrad stage could be calculated the footnote of Table 2. For a more complete de- scription see SARVAS(1972). Careful simulation under natural conditions also calls for monitoring temperatures locally at treetop-level and frequent 2'0" 25" 30" sampling of generative buds (SARVAS1972; 1 LUOMAJOKI1977, 1982). 69' The timing of the successive stages of tree meioses was studied in calendar time by LUOMAJOKI (1982). A more physioecological view was then 68" taken by LUOMAJOKI(1984) in terms of simulation the meiotic development in degree-day and period 67" unit heat sums. It was not only found that the male 67" generative development in the spring followed closely the temperatures, but that the so called zero 66" point problem was the worst source of error in the 66"- simulation of development. While the simulation of development itself was rather accurate, the yearly p.u. (period unit) sums covered periods of winter 6 5' dormancy as well as the "active period" (see SAKVAS 1972) of development sought after. The photoperiod has been shown to have an in- 64' creasing influence on the timing of various vegeta- tive phenomena towards the end of the growth peri- od (DORMLINGet al. 1968; SMITHBERGand WEISER 1968; ROCHE1970; ANDERSON1974; HABJ0RG 1978; FUCHIGAMIet al. 1982; JUNT~ILA1982; KOSKIand SEL- KAINAHO 1982; KOSKIand SIEVANEN1985). It is there- fore reasonable to consider the possible effects of the photoperiod on the generative development and on the timing of male meiosis in Betulaceae in par- 61' { ticular. The geographical and yearly variations in the tim- ing of male meioses in the spring were found to be very large (LUOMAJOKI1984). In the same year the meioses in northern stands of Finland lagged the meioses of southern stands by as much as four Fig. 1. The localities where microsporogenesis materials weeks. were collected. One to six experimental stands (see Table In this paper the relevant temporal variations in 1) were studied at each locality. These places were in lati- Betulaceae will be appraised and compared to tudinal order: 1 Bromarv (60'02'; annexed in 1977 toTen- hola parish), 2 Tuusula (60"21'), 3 Punkaharju (61"48'), 4 species previously studied. Is the variation equally Kerimaki (61"51'), 5 Viippula (62"03'), 6 Rovaniemi rnlk. large, and are the geographical correlations similar (66"21') and 7 Inari (69"04'). The distance between locali- to meioses that occur in the spring? The relevant ties 1 and 7 is ca'. 1050 krn. Localities 1-5 were grouped to- physioecological factors, daylength and tempera- gether as southern Finland and localities 6 and 7 as ture, are evaluated as far as possible. Is the temper- northern Finland. Hereditas 104 (1986) VARIATIONIN TIMINGOF MICROSPOROGENESIS 233 Tab/? 1. Origin, age and elevation of the sample stands Species Site, plot Origin Age in 1970 Elevation (years) (4 A/nus glic/rnosa Tuusula3 Local 100 50 A. iticana Inari, Toivoniemi' Local Ca. 50 148 Kerimaki 540' Local 52 101 Punkaharju LXII Local 44 81 Tuusula XLI Local 34 40 Betuh pendula Bromarv VI Local Ca. 40 5 Kerimaki 5432 Local 66 86 Punkaharju LIV Local 59 90 Rovaniemi XXVlIl Local 44 251 VilppulaV Local 38 150 Inari. Toivoniemi' Local Ca. 40 156 Punkaharju XIV Local 43 85 Punkaharju L" Local 70 90 Punkaharju LX Local 38 83 Rovaniemi XVII Local 123 170 Tuusula 12 Local 52 45 Vilppula 153 Local 68 120 Corylrc uveNaria Bromarv v Local Variable 7 Punkaharju' Finland, Bromarv Unknown 90 I Nut it regular, numhered plot. The numbered plots are sample stands of Dept. Silviculture. Finnish Forest Research Institute 'The srnnd wacut down: Kerimaki 540 in 197S77. Kerimhki 543 in 1978, Punkaharju L in 1974-75 Material and methods The material was mainly collected between July meters below the treetop-level, or in all from a 1964 and August 1971 and it was extended in July population of ten trees, daily, 1-6 times a day, de- and August 1983. The seven localities where collec- pending on air temperatures. At Punkaharju 21 tions were made range from Bromarv (60'02') in the meter hydraulic aerial ladders on a tractor were very south to Inari (69'04') in the north (Fig. 1). This useful. At other locations conventional ladders study concentrates on Betula pendula Roth and Be- were used. tulapubescens Ehrh. Ahus incana (L.) Moench was The catkins were sliced (one half being discarded) also studied both in the north and in the south of and put in a fresh fixative of 25 vol. Yo of glacial ace- Finland. Alnus glutinosa (L.) Gaertner and Corylus tic acid and 75 vol. % of absolute ethanol. (This fix- avellana L. were studied only locally (names are ac- ative contains less acetic acid than that used for con- cording to Flora Europaea, TUTINet al.