partial reproductive incompatibility between populations of spider mites (acarina: tetranychidae)

Ent. exp. & appl. 20 (1976)225--236. North-Holland Publ. Co. Amsterdam

PARTIAL REPRODUCTIVE INCOMPATIBILITY BETWEEN POPULATIONS OF SPIDER MITES (A CARINA:

TETRANYCHIDAE )

BY

W. P. J. O V E R M E E R and A. Q. VAN Z O N

Laboratory of experimental Entomology, University of Amsterdam, The Netherlands

Partial reproductive incompatibility between spider mite populations of different origin is a common phenomenon. A comparative study was made of the characteristics of partial reproductive incompatibility both between various natural populations of Tetranychus urticae and between a laboratory strain and a number of chromosome mutation homozygous strains derived from it. lnterpopulation crossing experiments were set up and percentages of nonviability assessed in the F1, the haploid F2, the Bj obtained from hybrids that had been mated with males of the original male parent stock and the B 1 produced by hybrids that had been mated by males of the original female parent stock. In those cases where partial reproductive incompartibility between populations is due to different chromosome mutations similar degrees of nonviability were observed for the haploid F 2 and the B1 generations belonging to it. With natural reproductive incompatibility the degree of lethality was highest in the haploid F2; the percentage nonviability was usually less for fertilized eggs, and especially when the eggs were fertilized by a gamete of a male of the same origin as the female parent of the hybrid. Differences in hybrid sterility between reciprocal crosses were often found, it is assumed that, although chromosome mutations may play a role in speciation of tetranychids, partial reproductive incompatibility between natural populations is mainly due to lethal recombinations of genes and interactions between cytoplasmic factors and genes of "alien" male gametes.

Outcross ing exper iments be tween different spider mite popula t ions o f the same species, of ten reveal partial reproduc t ive isolation be tween such populat ions . Dec reased genet ic compat ibi l i ty be tween different popula t ions within species has been repor ted for Tetranychus urticae (Boudreaux , 1963; Helle & Pieterse, 1965), T. pacificus (Bal lantyne, 1969), T. neocaledonicus (Gut ier rez & Van Zon, 1973), T. lombardini (Helle & Overmeer , 1973) and Panonychus citri (Tanaka et al., 1969). Moreover , cases o f comple te in terpopula t ionai reproduc t ive incompat ibi l i ty within a species or species complex have been found (Helle & Van de Bund, 1962; Die leman & Overmeer , 1972; Gut ie r rez & Van Zon, 1973; Smith, 1975). The existence of such incompat ibi l i ty led to the possibility o f applying genet ical control p r o g r a m m e s against injurous spider mite species by releasing " i n c o m p a - t ible" males (Helle, 1969; Smith, 1975).

Little is known about the principles under lying the in terpopula t ion partial reproduc t ive incompatibi l i ty . Boudreaux (1963) suggested that t rans locat ions and inversions might be o f impor t ance in this respect . This hypothes is was based on the fact that it is general ly the F~ of in terpopula t ion crosses that is affected, i.e. a

226 W . P . J . OVERMEER AND A. Q. VAN ZON

proportion of the eggs laid by the F~ do not hatch, whereas in the first instance a viable F 1 is obtained. Gametogenesis in the F~ is apparently disturbed. Helle & Pieterse (1965), however, also mentioned the incidental occurrence of lethality in the F 1 of a cross between two populations of T. urticae, and similar observations have also been made for T. neocaledonicus (Gutierrez & Van Zon, 1973). Helle & Overmeer (1973) supported Boudreaux's idea of chromosome polymorphism being the basis of decreased reproductive compatibility, as Van Zon & Overmeer (1972) were able to produce various chromosome mutation homozygous strains of T. urticae which showed a reduced genetic affinity with each other. These mutations were induced in a normal laboratory strain of T. urticae with the aid of X- irradiation. The mutations concerned were in all probability reciprocal translocations (see Pijnacker & Ferwerda, 1976).

Overmeer (1976), however, recently doubted if translocations or other drastic chromosomal rearrangements are involved in naturally occurring partial reproductive incompatibility between populations, since the induction of structural chromosome mutations in a given normal population unequivocally resulted in an increased degree of reproductive incompatibility towards other natural T. urticae populations.

The present paper is a genetic study of partial reproductive incompatibility between poPulations of T. urticae. Crosses were made between different glasshouse populations, and the lethality in successive generations was compared with that observed in crosses between several chromosome mutation strains of T. urticae and the normal strain in which the chromosome mutations had been induced. The investigation was designed to increase understanding of the principles underlying speciation in the tetranychids.

MATERIAL AND METHODS

The following strains of T. urticae were used. - - WE, a white eye laboratory strain which has been maintained in the laboratory

since 1969. - - B, H, J and K, chromosome mutation homozygous strains, obtained following

X-irradiation (2,25 kr) of WE males (Overmeer, 1976). - - vE, Ps, Nd, Tp, GV, Vr, Dw, vL and HV, strains of different glasshouse

populations which have been in culture in the laboratory since 1973.

Mass crossing experiments were carried out on detached bean leaves on wet cotton wool. Female teleiochrysalids of a given strain were isolated with adult males from another strain. Once the females started laying eggs, they were allowed to oviposit for a period of two days; thereafter females and males were removed. The F~ was examined with regard to nonviability. From each cross a number of hybrid females were collected, still in the teleiochrysalis stage, and distributed over three new bean leaves. On the first leaf the females were not mated and therefore produced only haploid eggs. On the second leaf the hybrid females were backcrossed with males of the original paternal strain, and on the third leaf the

I N C O M P A T I B I L I T Y B E T W E E N S P I D E R MITE P O P U L A T I O N S 227

hybrid females were backcrossed with males of the original maternal strain. In the latter two cases the eggs consisted of a mixture of haploid and diploid eggs. About 30 to 50 females were placed on each leaf and in case of backcrossing 15 to 25 males were added. The hybrid females oviposited for 2 days but in those cases where the number of eggs produced was relatively small, viz less than 300 eggs, the females were allowed to stay on the leaf for another day. The percentage nonviability was assessed after 7 days, i.e. before adults emerged, thus information was obtained about the degree of nonviability of pure haploid offspring and of mixtures of haploid and diploid offspring. The nonviability is principally expressed by the failure of the egg to hatch. Survival of progeny from hatched eggs was usually good.

Whether or not differences in partial hybrid sterility occur between reciprocal crosses is best studied by comparing the percentages nonviability of the haploid F 2 of the reciprocal crosses. Haploid nonviability is a better graduator than nonviability of haploid and diploid progeny as the ratio haploid/diploid may vary in the progeny, and the degree of nonviability of haploids may considerably differ from that of diploids. The influence of mating can be determined by comparing the percentage nonviability of the haploid F 2 and that of the B l of the same original cross.

For some crosses the pure diploid mortality in the B l generations was estimated by counting the number of viable females and estimating the fraction of nonviable diploid eggs, i.e. the total number of nonviable eggs minus the nonviable haploid ones. The number of lethal haploid eggs was calculated by counting the number of viable males present in the Bt concerned and multiplying this number with the ratio nonviable/viable haploids observed in the corresponding haploid F 2 (cf Feldmann, 1975).

For two crosses the individual variation with regard to the degree of hybrid sterility was examined. It concerned the cross WE 9 9 x J 6 ~ , in which semi- isolation was due to artificially induced structural chromosome mutations, and the cross WE Q 9 x Ps c3c3, a case of natural decreased compatibility. Hybrid sterility was expressed by the percentage non-hatch of the haploid eggs produced by these hybrids. The hybrids were assayed individually. Thus, mass crosses were set up between the stocks involved and the hybrids obtained placed individually on separate bean leaves, where they deposited eggs for 4 days. They were then transferred to new leaves where they oviposited for a further 4 days. The degree of hybrid sterility in each individual was determined using samples of 120 hybrids of each of the two crosses. Hybrids were chosen from those that had survived at least the first 4-day-period.

RESULTS

The F l of reciprocal crosses between WE and the chromosome mutation strains was not markedly affected. The percentage nonviability was always less than 7%. Data on the nonviability in the F 2 of these crosses and the B l generations are given

228 W.P . J . OVERMEER AND A.Q. VAN ZON

in Table I. The results are presented so that data concerning reciprocal crosses can easily be compared: the crosses WE Q Q • chromosome mutation strain (3 c~ in the upper half of the table and reciprocal crosses, viz chromosome mutation strain Q ~? x WE c3 ~ in the lower half. Confidence limits at 95% level are given. In all B~ samples the proportion of females was found to exceed that of the males, thus mating had been effective.

TABLE 1

Percentages of nonviability in the haploid F~ and 1st backcross generations of reciprocal crosses between WE and chromosome mutation homozygous strains o f T e t r a n y c h u s ur t icae . The three values beneath each other refer respectively to the haploid F 2, the B j whereby hybrids were mated by males of the paternal type and the B whereby hybrids were mated by males of the maternal type. Number of eggs per sample is more than 500. 95%

confidence limits are given in parentheses

B H J K

WE Q q? 46(42--50) 76(72--80) 74(70--78) 63(59--67) 47(43--51 ) 76(72--80) 68(64--72) 65(61 --69) 44(40--48) 75(71--79) 66(62--70) 62(58--66)

WE 6 6 45(41--49) 74(70--78) 75(71 --79) 61 (57--65) 44(40--48) 75(71--79) 75(71--79) 59(55--63) 44(40--48) 75(71--79) 73(69--77) 62(58--66)

Two facts are apparent: there are no clear differences between reciprocal crosses and mating does not markedly influence the degree of nonviability. The percentage nonviability for haploids and diploids is approximately the Same. Apart from the case (WE x J) 9 x J c3 the percentage nonviability of diploids and haploids does not differ significantly from the percentage nonviability of the haploids. The difference between 66% (size of the sample was 471 eggs) and 74% (sample 503 eggs) is significant, ~ = 6.39 p = 0.011. However, the values for lethality are of the same order of magnitude.

The data on nonviability in succeeding generations of crosses between WE and each of the glasshouse strains are given in Table II (top row and extreme left row) and Table III. The Fj of the various crosses was not severely affected, with the percentage nonviability ranging between 5 and 10% (Table II). The sex ratio in the various F 1 samples was always in favour of the samples, viz more than 1.8 female to 1 male. Nonviability with regard to haploid F 2 and B~'s of the various combinations are presented in Table III. 95% confidence limits are given. The sex ratio was in all BI samples in favour of the females. The estimated percentage nonviability of the diploid progeny in the various samples is noted in brackets after the corresponding percentages for the haploid-diploid mixtures. For this series of crosses differences in nonviability of the haploid F 2 occur regularly between reciprocal crosses. Only for the reciprocal crosses between WE and HV there is no significant difference between the percentages nonviability of haploid F~. For all others there is a significant difference (95% confidence intervals do not overlap) however, with regard to the crosses between WE and Nd the difference is relatively small.

INCOMPATIBILITY BETWEEN SPIDER MITE POPULATIONS 229

TABLE [I

Percentage nonviability in the F 1 of crosses between various glasshouse strains o fTe t r anychus urticae and WE. Number of eggs per sample is more than 350

c~ Q 6" d v E Ps Nd Tp GV Vr Dw vL HV WE

vE 5 5 5 6 5 5 6 4 5 5 Ps 6 4 4 5 7 6 5 5 6 5 Nd 4 5 4 5 5 4 5 4 5 6 Tp 6 6 3 6 5 5 5 8 5 5 GV 5 4 2 5 4 4 6 9 12 7 Vr 5 4 5 2 3 4 5 6 9 6 Dw 6 5 8 6 4 3 6 7 5 7 vL 4 5 6 10 5 5 6 3 7 5 HV 8 7 7 5 11 11 7 8 3 6 WE 6 8 7 8 9 9 6 7 6 2

Note: the 95% confidence interval for 2% nonviability (sample size 350 eggs) is I---4%, and for 12% the 95% confidence interval is 8--16%.

The percentages of nonviability of the F 1 between a particular glasshouse strain and the other glasshouse strain are given in Table II. The nonviability in the various samples did not exceed 12%. The sex ratio in the various samples ranged between 1.7 females to 1 male and 2.4 females to 1 male. The size of each sample was more than 350 eggs. The percentages of nonviability in haploid F 2 and Bj generations are given in Tables IV to XII. The size of the various samples varied between approximately 300 eggs and 600 eggs. Confidence limits are not presented to keep the tables readable. The variation of the degree of nonviability of progeny from mass crosses is presumed to be small, so that the percentages presented can be considered as rough but valid information. Differences in reciprocal crosses were found in at least 35% of the crosses studied. Mating generally reduces the percentage of nonviable eggs produced by the hybrids especially when hybrids were mated by males of the same type as the original female parent.

In general, the lower haploid F~ nonviability percentages correspond to lower F I nonviability values, viz between 5% and 8%, and the higher values, such as 80% or more, of haploid F~ nonviability correspond to F I nonviability values of 9% or more. The lethality mainly affected the eggs. Nonviable larvae were found only incidentally.

The variation of the degree of hybrid sterility for the cross WE Q Q x J c3 6 was measured by examining 120 hybrids. The degree of hybrid sterility, expressed by the percentage nonhatchability of the haploid eggs produced by each female, ranged between 65 and 88%. Mean and standard deviations were found to be 77 and 5%, respectively. As for the 120 hybrids of the cross WE Q Q x Ps c3 z~ sterility percentages were between 35 and 100%, with a mean of 71% and a standard deviation of 12%.

230 W. P, J. OVERMEER AND A. Q. VAN ZON

TABLE III

Percentages of nonviability in the haploid F 2 and 1st backcross genereations of crosses between WE and glasshouse strains ofTetranychus urticae. Presentation see Table I. The number of eggs per sample is more than 430. 95% confidence limits are given in parentheses for the haploid F v The estimated percentages of nonviabi-

lityfor diploid eggs are given in brackets

W E q ) ~

WEe36

WE vE Ps Nd Tp GV

4 66 (62--70) 70(66--74) 83 (79--86) 69 (65--73) 92(88--95) 58 [53] 67 [661 80 [77] 64 [58] 86 [82] 31 [20] 28 [81 50 [2] 45 [38] 57 [35]

4 46 (42--50) 29 (25--33) 74 (70--78) 42 (38----46) 62(58--66) 33 [28] 28 [281 60 [53] 35 [35] 51 [48] 24 [211 16 [12] 38 [191 24 [22] 26 [111

W E C ~

WE c~ C~

Vr Dw vL HV

4 84(80--87) 80(76--83) 68 (64--72) 58(54--62) 84 [84] 75 [701 66 [65] 53 [51] 28 [8] 66 [60] 50 [32] 23 [11]

4 65 (61--69) 58 (54--62) 53 (49--57) 59 (55--63) 55 [491 46 [40] 47 [43] 50 [49] 30 [11 55 [52] 22 [10] 17 131

TABLE IV

Percentages of nonviability in the haploid F 2 and 1st backcross generations of reciprocal crosses between vE and the other glasshouse strains ofT. urticae. Presentation see Table L Size of the samples 300--600 eggs

v E g Q

vE Ps Nd Tp GV Vr Dw vL HV

7 15 26 27 20 56 58 51 16 10 20 18 20 40 51 40 20 5 12 12 12 25 20 42 23

7 21 6 26 12 16 60 48 60 24 7 20 6 14 51 51 32 21 13 14 9 9 49 20 30

v E 6 6

TABLE V

Percentages of nonviability in the haploid F 2 and 1st backcross generations of reciprocal crosses between Ps and the other glasshouse strains ofT. urticae. Presentation see Table L Size of the samples 300--600 eggs

Ps (2c2

Ps6 6


21 7 15 17 65 56 65 60 67 24 14 16 46 56 56 58 64 21 t3 10 32 22 56 40 48

15 7 15 30 16 20 65 55 77 10 15 18 19 19 57 49 75 5 11 13 8 19 55 42 55

r~

7 ,

Z

Z

>

232 W.P.J. OVERMEER AND A. Q. VAN ZON

TABLE X

Percentages of nonviability in the haploid F 2 and 1st backcross gc:wrations of reciprocal crosses between Dw and the other glasshouse strains ofT. urticae. Presentation see Table L Size of the samples 300--600 eggs

Dw 9 9

D w ~ 6


60 65 81 45 50 48 8 65 55 51 57 61 30 42 15 66 49 49 55 68 30 19 18 37 49

58 65 53 51 63 42 8 76 73 51 56 44 20 31 18 70 75 20 56 48 16 15 18 48 56

TABLE Xl

Percentages of nonviability in the haploid F~ and 1st backcross generations of reciprocal crosses between vL and the other glasshouse strains ofT. urticae. Presentation see Table I. Size of the samples 300--600 eggs

v L 9 9

vLc~ ~'


48 55 65 85 54 34 76 5 72 51 49 40 60 45 29 70 70 20 22 41 40 21 16 48 36

51 60 61 97 89 74 65 5 82 40 58 57 87 83 50 66 83 42 40 40 50 31 57 37 26

TABLE XII

Percentages of nonviability in the haploid F 2 and 1st backcross generations of reciprocal crosses between HV and the other glasshouse strains ofT. urticae. Presentation see Table 1. Size of the samples 300--600 eggs

H V g Q

H V 6 6


60 77 67 43 100 95 73 82 4 32 75 76 52 96 92 75 83 30 55 53 42 86 73 56 26

16 67 63 30 100 90 55 72 4 20 64 54 29 92 82 49 70 23 48 57 30 81 62 49 36

DISCUSSION

In the present study the interpopulation partial reproductive incompatibility always expresses itself as hybrid breakdown. Hybrid females were always obtained but part of their offspring was inviable. In general, the F 1 showed a normal viability, though in a few cases the F 1 mortality differed significantly from the average intrapopulation check mortality, which is approximately 5% (Table II). In case of the cross GV Q 9 x HV 5' c3, for instance, the percentage nonviability was 12%. In that particular offspring the sex ratio was approximately 2 females to 1 male. If one assumes that the percentage nonviability for haploid GV eggs is no


more than 5%, it means that the percentage nonviability of the diploid offspring of GV Q q? x HV 6 6 is noticeably more than 12%, namely 15 or 16%. This percentage is the highest value for hybrid inviability observed in the present study. If one compares these results with those obtained by Helle and Pieterse (1965) for T. urticae where in one case 55% Fj nonviability was found, or with the results for T. neocaledonicus (Gutierrez & van Zon, 1973) where in two instances on F I nonviability of more than 70% was found, one may state that in the present study the F~ was not or only slightly affected.

Although, as was mentioned before, reproductive incompatibility expresses itself as hybrid breakdown, a comparison of Tables I and III, for instance, shows that clear differences exist in the patterns of nonviability. For the chromosome mutation strains there are no differences in the reciprocal crosses; this is not true for the different glasshouse populations where large differences in nonviability can be seen. Table I shows that there is no effect of mating on the percentage of nonviability whereas in Table III large differences following mating are frequently observed. These different patterns of nonviability must indicate a difference in the basis of incompatibility. If one further compares the variation of the degree of hybrid sterility for the cross WE Q Q x J ~ ~ with that of the cro~s WE q? Q x Ps

~', the different results again suggest a fundamental difference in the mechanism of incompatibility.

From Tables IV to XII it seems that the patterns of nonviability are more in accordance with the pattern shown in Table III than with the data presented in Table I. Table X seems closest to the pattern found in Table I. Yet, in all Tables IV--XII increase of egg-viability is noticeable when the eggs were fertilized. This indicates that the results shown in Table III are not characteristic for WE only. In cases where the degree of hybrid sterility is relatively low, i.e. in mutual crosses between vE, Ps and Nd (Tables IV, V and VI), nevertheless, it is more difficult to demonstrate a distinct difference in the percentage lethality between haploid sanlples and corresponding haplo-diploid ones.

In case of semi-isolation due to chromosome mutations, the lethality is due to the formation of aneuploid gametes. The frequency of unbalanced gametes produced is not expected to be different for hybrids of reciprocal crosses. Both types of hybrids are similar with regard to genotype and cytoplasm, as the chromosome mutation strains were derived from WE. Lethality due to unbalanced gametes is apparently a dominant character, as mating does not noticeably reduce the percentage nonviable eggs produced by the hybrid.

The question arises whether F 2 inviability due to chromosome polymorphism is always dominent. Chromosome polymorphism in the chromosome mutation strains under study is a consequence of X-irradiation resulting in rather drastic rearrangements. Pijnacker & Ferwerda (1976) have shown that reciprocal translocations in the J strain have taken place in all three chromosomes. It is possible that less drastic reshufflings will produce aneuploid gametes with only minor deletions, which might be genetically saved by a normal male gamete, although such gametes are not viable when they remain unfertilized.

234 W . P . J . OVERMEER AND A. Q. VAN ZON

In natural partial reproductive incompatibility chromosomal reshuffiings may play a role. These mutations are of importance in speciation in general (White, 1957; Smith, 1960; Da Cunha, 1960). Yet they can not be entirely responsible for the degree of nonviability observed in the present study on spider mites. Mating generally decreases the percentage nonviability of eggs, but, as was mentioned before, especially when the males are of the same origin as the original female parent. Consequently, there must be another principle involved. Lethality supposedly results from nonviable recombinations of genes or perhaps even whole chromosome segments. Certain recombinations must be imbalanced with regard to genetic regulation~ i.e. the possibility of controlling a particular biochemical pathway is disturbed. Recombination of certain genes deranges or desynchronizes the synthesis of vital proteins, particularly in the haploid phase. Fertilization by a balanced gamete may partially restore the genetical damage. In the F 1 female there are two complete balanced sets of chromosomes and apparently there is normally no interaction between homologous genes or chromosome segments since the F1 is generally viable. In other cases, however, there may be an interaction (c.f. Helle & Pieterse, 1965; Gutierrez & Van Zon, 1973).

There is another aspect to be considered. For at least 40% of the cases in the present study differences were found in the degree of hybrid sterility for reciprocal crosses. This points to a maternal effect; the cytoplasm or cytoplasmic factors must play a role. The fact that mating with males of the original maternal strain reduces the percentage nonviability of the hybrid eggs considerably more than in mating with males of the original paternal strain underlines this. An interaction between "alien genome" and cytoplasmic determinants seems likely. The interaction, on the other hand, is such that it does not affect the viability of the F~, apparently because there is already one complete set of chromosomes available that is compatible with the cytoplasm. Male gametes of the original female parent are tolerated by the cytoplasm, and therefore potentially able to restore the viability of recombinants in the F 2 which are lethal in the haploid stage. The lethality of those diploid eggs that die might nevertheless be due to chromosomal polymorphism. This latter aspect is clearliest observable in Table X.

It is difficult to understand why both reciprocal crosses between HV and GV (Tables VIII and XII) resulted in 100% lethality of the haploid F~. One would expect at least some of the gametes produced to be sufficiently similar to the maternal genome type which would fit with the cytoplasm. Still, part of the lethality can be suppressed by mating.

If one considers the various data on reproductive isolation in spider mites showing frequently differences between reciprocal crosses, it appears that cytoplasmic determinants are an important element in the genetic system of these mites. As a consequence, it seems possible that the presence of cytoplasmic determinants may somehow be related to sex determination. In this respect it is of interest to examine, how far Goldschmidt's theory (1934) for the sex determination of the honey bee, might be of value for spider mites. In this case maleness determining factors (M) are supposed to be extrachromosomal and femaleness


d e t e r m i n i n g f a c t o r s ( F ) a r e c h r o m o s o m a l . T h e c o m b i n a t i o n s M F ( m a l e ) a n d M F F

( f e m a l e ) m i g h t we l l e x p l a i n t h e d e t e r m i n a t i o n o f sex in s p i d e r m i t e s . I n c o m p a t i b i -

l i ty o f F w i t h M m i g h t l e a d t o l e t h a l i t y . M o r e d e t a i l e d i n f o r m a t i o n is n e c e s s a r y in

t h i s r e s p e c t .

ZUSAMMENFASSUNG

TEILWEISE FORTPFLANZUNGSUNVERTR,~'GL1CHKEIT (INKOMBATIBILIT.~'T) ZWISCHEN POPULATIONEN VON SPINNMILBEN (ACARINA : TETRANYCHIDAE)

Teilweise Fortpflanzungsunvertr,~glichkeit zwischen Spinnmilbenpopulationen verschiedener Her- kunft ist ein h~tufiges Ph~,nomen. In einer vergleichenden Studie wurden die Eigenschaften solcher lnkombatibilit~it sowohl zwischen natfirlichten Populationen yon Tetranychus urticae als auch zwischen einem Laborstamm und einigen von diesem hergeleiteten St~tmme mit homozygoter Chromosomen- mutation untersucht. Kreuzungsexperimente zwischen den Populationen wurden durchgeffihrt und Prozents/itze der Nichtlebensf~,higen (= nicht schlfipfenden Eiern) bestimmt u.zw. in der F 1, der haploiden F 2, der B , die von Hybriden erhalten wurde, die mit M~mnchen der ursprfinglichen Elternzucht ffir MS.nnchen gepaart waren und schlieglich der B , von Hybriden erzeugt, die gepaart waren mit Mfinnchen der ursprfinglichen Elternzucht f/Jr Weibchen. In den FS.11en, wo die Inkombatibilitfit zwischen den Populationen verursacht wird durch verschiedene Chromosomenmuta- tionen, wurde ein ~ihnliches Ausmag der NichtlebensfS.higkeit beobachtet ffir die haploide F 2 und die dazugeh6rigen Bj-Generationen. Bei natfirlichen teilweiser FortpflanzungsinkombatibilitS.t war d a s Ausmag der Lethalitfit am h6chsten in der haploiden F2; der Prozentsatz Nichtlebensffthigkeit war gew6hnlich ffir befruchtete Eier geringer, besonders wenn die Eier befruchtet waren dutch einen Gameten von einem Mfinnchen der gleichen Herkunft wie der weibliche Elternteil des Hybrids. Oft wurden Unterschiede in der HybridsterilitS.t zwischen reziproken Kreuzungen gefunden. Es wird angenommen, dag, obwohl Chromosomenmutationen eine Rolle spielen m6gen in der Speciation der Tetranychiden, teilweise Fortspflanzungsinkombatibilit/it Zwischen natfirlichen Populationen haupt- sftchlich verursacht wird dutch letbale Rekombination von Genen und Wechselwirkungen zwischen zytoplasmatischen Faktoren und Genen yon , f remden" Mfinnchengameten.

REFERENCES

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Receivedforpublication: April 24, 1976

partial reproductive incompatibility between populations of spider mites (acarina: tetranychidae)

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