between them is known te be possiblet and the resultant

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H. Akai and M. Kiuchi, eds.

Effects of photoperiod on the duration of pupal stage of the wild silkworm, Bombyx mandarina Moore

Jun Kobayashi

Department of Insect Genetics and Breeding National Institute of Sericultural and Entomological Science

Tsukuba, Ibaraki 305, Japan

1. IN!I!RODUCTION

The wild silkworrn, !B2g!!!2y2smb mandarina, and the dornestic

silkwormt E. mori, are closely related species. Coupulation

between them is known te be possiblet and the resultant

progenies are never sterile (e.g., 1-4).

Only a little is known about the life cycle of E. man- darina. Ohba (5) and Ohmura (6) reperted that the duration of hatching frorn diapausing egg and the duration of pupal stage of E. rnandarina were longer and more variable than those of P.

mori. However, environmental factors which affect the life cycle of l}. mandarina are still unknown. RecentZy I reported that female moths of B. mandarina produced diapausing eggs when they were reared under short-day photoperiod during the Zarval stage, like the tropical races of E. mori (7).

Zn this paper ! tiill describe effects of photoperiod on the pupal duration of E. mandarina and the relationship be- tween the pupal duration and the inductien of egg diapause.

Table 1. Sites of collecting geographical strains of B. rnandarina examined in this study Geographical

strain Collecting site Latitude

Sakado Sakado, Saitama, Japan 36.00N

To-shirna To-shima rsland, Tokyo, Japan 34.50N Kozu-shima Kozu-shima rsland, Tekyo, Japan 34.20N

Hangzhou Hangzhou, Zhejiang, China 30.20N

57

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2. MATERIALS AND METHODS

Four geographical strains of E. mandarina were used in this experirnent. One of thern was collected in China and the others in Japan. Collecting sites are shown in Table 1.

Each strain was kept in an insect rearing charnber

(Shimazu Co.t Ltd.) frorn egg incubation until adult ernergence.

Four photoperiodic regimes were estab!ished for this experi- ment. [Dwo of them were constant photoperiodst long-day (15L9D) and short'day (12M2D). The others were photoperiodic changest long-day (15L9D) to short-day (12L12D) and vice versa. rn the case of photoperiodic changes, the 3rd instar larvae just

after the 2nd larval ecdysis were transferred from one

photoperiod to the other. Ternperature was kept at 250C during this experiment. Larvae were reared on mulberry leaves.

3. RESUI,TS

Photoperiodic respenses of each strain are shown in Fig.

1 and Table 2. The pupal duration of the Sakado strain varied with photoperiodic regimes (Fig. Ia). In both rnale and femalet the means of pupal duration were the shortest (18-19 days) un- der long-day (15L9D) photoperiod and became longer (25-27 days) under short-day (12L12D) (Table 2). 'Furtherrnore, under the photoperiodic changes (15L9D-12L12D and 12L12D-15L9D) the means became the longest values (28-30 days). Because the dis- tributions of the pupal duration were very wide (13-41 days) under these changes, the standard deviations also becarne

larger (4-9 days). '

Photoperiodic responses of the To'shirna strain and Kozu- shima strain were different from the Sakade strain. rn these strainst most of pupal durations were longer than.20 days and the distributions of the pupal duration were relatively wide under all the photoperiodic regimes (Fig. Ib-c). Therefore, the means and the standard deviations of the pupal duration under each photoperiodic regimes were as large as those of the Sakado strain under the photoperiodic changes (Table 2).

Although the Chinese strain, like the Sakado straint showed shorter rnean of the pupal duration (17 days) under 15L9D and longer mean (22 days) under 12L!2D, this strain showed the shortest mean (15 days) under 15L9D'12L12D and longer rnean (22-23 days) under 12L12D-15L9D (Table 2). These resuZts strongly suggest that the rnost sensitive stage to photoperiod of this strain is in a certain stage prior to the 2nd larval ecdysis. !n this strain.there was no individual

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whose pupal duration was more than 30 days (Fig. Id).

rn Fig. 2, the relathionship between the pupal duration of fernaies and the induction of egg diapause was summarized.

Under 15L9D-12L12D, fernale moths of the Sakado strain ex- perienced shorter pupal durations produced non-diapausing eggs and those experienced longer pupai durations produced diapaus"

ing eggs (Fig. 2a). Under 12L12D-15L9D, however, most of fernale moths experienced ionger pupal durations produced non- diapausing eggs. From this disagreernent between the pupal duration and the induction of egg diapause, it is obvious that these two deveiopmental phenornena are independently controlled

rn the To-shima strain and Kozu-shima strain, because the pupai duration was not affected by photoperiod, there was no obvious relationship between the pupal period and the induc- tion of egg diapause (Fig. 2b-c).

Zn the Chinese strain, female rnoths experienced shorter pupal durations (Åq19 days) produced non-diapausing egg and those experienced longer pupal durations (År19 days) produced diapausing eggs (Fig. 2d). This agreernent between these two developrnental phenomena was considered as a reflection of the coincidence of their rnost sensitive stages to photoperiod.

4. DrsevssloN

Tt is well known that the wild silkworm, !Bsgy2Åresomb x mand darina, hibernates at the egg stage and there 'is no observa- tion on the overwintering at the pupal stage in the field.

Ohba (8) reported that all the pupae collected in September had become rnoths before December. !n this study, although the Sakado strain and the Chinese strain showed longer pupai dura- tions under 12L12D than under 15L9D, the longest duration un- der 12L12D was about 30 days and seemed not to be sufficient for overwintering. Therefore, it is strongly suggested that

the pupaX stage is not the overwintering stage of !Bsgy2År[2smb x rnan- darina. rf there are sorne ecological roles in the long pupal duration, they must be played frorn spring to auturnn.

Ohrnura (6)t who reared E. mandarina in the laboratory and observed its long and variable pupal durationt inferred that l}. mandarina might have a complex voltinisrn: the rnajority of individual within the same population trivoltine, some of them bivoltine and a few univoZtine and tetravoltine. He thought that this compiex voltinism were rnainl'y caused by the long and variable pupal duration. In factt any tirne from June to Sep-

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Table 2. Effects of photoperiod on the pupal duration of geographicaX straÅ}ns of l}. mandarina

Pupal duration (meanÅ}SD) Geogra- ---.-..----....-.----"-"----."--

phical Photoperiodic regirne

15L9D 12L12D -12L12D -15L9D

Japan Sakado To- shirna Kozu-' shima China

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tembert alrnost all stages of E. ruandarina can be observed in a rnulberry field. This fact is one of proofs that the population of E. -andarina consists of different generations.

Zntrapepulation variations in insect seasonal cycles are frequently observed in the field and some of thern appear espe' cially important in spreading over time the risks involved with terrttinating dormancy and initiating growth, developrnentt and reproduction in an unpredictable environrnent (9). There- foret those long and variable pupal durations in the To-shima strain and Kozu-shima strain (Fig. Ib-c) might reflect unpre- dictable, environmental changes in these islands. The Sakado

strain ' also exihibited long and variabie pupal durations under the photoPeriodic changes (Fig. Ia). rn this strain the sudden change in photoperiod might be recieved as a unpredictable en- vironment.

Zn this paper, it was re'vealed that there were dif- ferences in effects of photoperiod to the pupal duration among geographical strains of E. mandarina and that, at least in the Japanese strains, the pupal duration and the induction of egg diapause were independent phenomena. As yet, only little is known about the life cycie of E. rnandarina in the field. Thus, for further study of the functions and adaptive value of the long and variable pupal duration, it will be necessary to in' vestigate the wild population in detail.

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REF:E RENCES

1. Kawaguchi, E. (1928) Z. f. Zeilforsch. u. mikroskop. Anat.

7,519-552. (In Gerrnan.)

2. Minarni, S. & Ohba, H. (1933) J. Seric. Sci. Jpn. 4t170- 172. (In Japanese.)

3. Minarni, S. & Ohba, H. (1939) Bull. d. Soies Kinugasa. 395t 71-82. (In Japanese.)

4. Aratake, Y. & Kayamura, T (1972) J. Seric. Sci. Jpn. 42t 331-339. (rn Japanese with English sumary.)

5. 0hba, H. (1939) Bull. d. Soies Kinugasa. 396,115-123. (Zn Japanese.)

6. 0tmura, S. (1950) Buil. Sericul. Exp. Sta. 13,79-130. (rn Japanese with English sumary.)

7. Kobayashi, J. (1989) in Wiid Silkrnoths '88 (Akai, H. & Wu, Z.S.,eds.), pp 73-78t rnternational Society for Wild Silk- rnothst Tsukuba.

8. 0hba, H. (1939) Bull. d. Soies Kinugasa. 397,201-202. (:n Japanese.)

9. Tauber, C.A., Tauber, M.J. & Masaki, S. (1986) Seasonal Adaptations of Insects. Oxford University Press, New York.

411pp.

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