Camp. Biochem. Phniol. Vol. 94A, No. 4, pp. 551-554, 1989 Printed in Great Britain
0300.9629/89 $3.00 + 0.00 c 1989 Pergamon Press plc
GENETICS OF THE GERMAN COCKROACH
MARY H. ROSS* and DONALD G. COCHRAN
Department of Entomology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. USA. Telephone: (703) 231-6341
(Received 22 Mq 1989)
The discovery that insecticide resistance is a heritable trait drew attention to a dearth of information on the genetics of economically and medically important insect pests. One of these was the German cockroach, Blattella germanica (L.). Genetic studies on this species were begun in our laboratory in the early 1960s in conjunction with research on insecticide resistance. Ease of rearing, a relatively low chromosome number (2n = 23 in male, 24 in females), chromosome specific identifications, and a gradual accumulation of morphological mutants encouraged development of a sustained program on the genetics of this species. Also, it seemed that this insect might have its own unique contributions to make to the body of knowledge on insect genetics. It is the only member of one of the lower orders of winged insects for which a formal genetics has been established. Other attributes are that embryonic development can be studied easily and post embryonic development can be followed from hatch through six nymphal instars into the adult stage.
By 1967, twenty-eight visible mutants had been isolated, five of the expected twelve linkage groups were tentatively established, and inheritance patterns of several insecticide resistance traits had been investigated (Cochran and Ross, 1967). By the time of a second review, the list of visible mutants had grown to fifty-eight, additional analyses of inheritance and linkage of insecticide-resistance traits had been completed, and research expanded to include cytogenetic studies (Ross and Cochran, 1975). Reciprocal translocations had been isolated and used to initiate linkage group-chromosome correlations and a system of length measurements for identifying meiotic chromosomes had been developed.
Presented here is a brief review of the current status of genetic studies on B. germanica, with emphasis on progress subsequent to the 1975 review. As indicated by the variety of topics covered, our research has broadened to include several aspects of genetics not previously studied.
*Author to whom all correspondence should be addressed.
AREAS OF RESEARCH
Formal genetics (mutants and linkage groups)
Three new mutants, large-body (Ig), miniature- wing (min), and maxillary-palp-elongate (mpe) have been added to stocks listed in the I975 review. lg has not been studied genetically, but it may be related to an extraneous segment of X-chromosome origin that is carried on a mid-length autosome in males drawn from the /g stock. This occurrence has implications for the evolution of B chromosomes in that it shows the origin of a heterochromatic segment that does not pair with a member of the normal complement.
min and mpe belong to a group of closely linked traits on chromosome 9 (linkage group VIII), includ- ing stumpy (sty), notched sternite (St), and pronotal winglets associated with a reciprocal translocation, T(9;10)/9;10 PHI, and with a deficiency, Df(9)Pw. Pw and st were the first mutants placed in this group and both are of evolutionary interest. Pronotal winglets resemble those on some of the earliest known fossil insects; st alters the development of ventral abdomi- nal segments in a primitive direction. Recent observa- tions of st include the occurrence of females with valvulae on a pregential segment and a late stage embryo carrying a pair of small segmented legs on the first abdominal segment, a position normally occupied by the primitive pleuropodium. In certain aneuploid eggs from backcrosses involving sf and a chromosome 9 translocation, segmented appendages occur on all abdominal segments. Presumably insects evolved from a multi-legged ancestor in which most segments were essentially identical. Structures on st individuals appear to represent an expression of a primitive developmental pathway.
min resembles sty in that it is a maternal-effect lethal and causes a similar alteration in the shape of the oothecae, but the traits are not allelic. An epistatic effect of min/f on pronotal winglets provides compelling evidence that there is a developmental pathway controlling wing development on all thoracic segments. Expression of this pathway in ancestral insects may have resulted in the pronotal winglets that characterize certain Paleozoic insects. Possibly a supressor, like that which apparently lies at the Pw+ locus, was responsible for the loss of pronotal winglets.
552 MARY H. Ross and DONALD G. COCHRAN
Studies on femur/tibia length ratios reveal a characteristic or sty that is intriguing in view of its close linkage with other chromosome 9 mutants that apparently express primitive developmental pathways (Tanaka and Ross, 1988). The ratios indicate a greater similarity between the three legs than in the wild type. They suggest that sty has primitive legs that are not fully differentiated from a hypothetical archetypal leg type that has been proposed on the basis of wild type regeneration studies. Regenerated sty legs are even closer to the proposed archetypal leg type.
mpe modifies the terminal segment of wild type maxillary palpi through a variable number of divisions that increase its resemblance to a thoracic leg (Ross and Tanaka, 1988). mpe, like S( and PLY,, causes a greater-than-normal identity of body parts. Involvement of these mutants in control of head, thoracic, and abdominal appendages, coupled with close linkage, suggests a relationship to the Drosophilu homeotic complexes. Nevertheless. mpe. st and Pii, differ from homeotic mutants in that they alter body parts so as to increase the resemblance to ancestral forms, including the appearance of structures in positions occupied by similar structures that disappeared during the course of evolution. In contrast. homeotic mutants cause a replacement of one body part by another with a similar developmen- tal or evolutionary origin. Perhaps the cockroach mutants should be regarded as primitive homeotics
The only additions to linkage groups since 1975 are pale-purple-eye (pp) to group VI and pearl eye (p), as well as mpe and min, to group VIII (chromosome 9). The linkage situation is still characterized by a concentration of known markers into three linkage groups. i.e. groups VI, VIII and X on chromosomes 8. 9, and 5, respectively.
The main thrust of genetic research of the German cockroach during the late 1970s and early 1980s was on chromosome identification, analyses of reciprocal translocations, and possible use of translocations for population control. Banding patterns in C-band and conventionally-stained preparations enhanced the earlier method of length measurement for the identifi- cation of late pachyteneeearly diplotene chromo- somes. For example, a translocation originally thought to involve chromosomes 2 and II (curly-wing) and another involving Nos 9 and 11 were reidentified by banding patterns as T(4:6)/4:6 Cu and T(8;9)/8;9, respectively, and linkage groupchromosome correlations revised accordingly. Patterns on other translocations used in linkage group correlations agree with earlier identifications by chromosome measurement. Consistency of banding patterns, also reflected in prophase II chromosomes (Barnes and Ross, unpublished data), is attibutable to the distribution of constitutive heterochromatin.
In translocation multivalents, a single chiasma is usually present near the end of each arm. This does not seem to involve a change in chiasma position, since wild type chiasmata appear to be localized distally (Keil and Ross, 1983). However, the occurrence of a chiasma in each arm of multivalents
does increase the mean chiasma frequency. For example, the mean frequency of wild type was estimated at 1.33 _+ 0.018 for first egg case progeny compared to 1.41 + 0.006 in the translocation heterozygote, T(8;9)/8;9. Chiasmata usually maintain quadrivalents as rings-of-four in diakenesis and metaphase I, greatly facilitating identification of adjacent and alternate orientations.
An alternate-2 orientation in an animal organism was first identified in B. germuniccl. In translocations characterized by random disjunction (50% alternate: 50% adjacent disjunction), orientation frequencies lit a 2 : 1: 1: 2 ratio of adjacent-l. alternate- I. adjacent-?, and alternate-2, respectively. A preponderance 01 alternate-2 orientations occurs in translocations characterized by a favoring of alternate over adjacent disjunction. Alterations through selection of a typical 70-72% alternate disjunction in T(3;12):3: I2 demon- strated that disjunction frequencies are under genetic control (Ross and Cochran. 1983).
Other characteristics of the translocations include close agreement between the frequencies of adjacent orientation and those of unhatched eggs that do not complete development (Ross and Cochran, 1983) Development generally ceases before formation of a segmented germ band. Since adjacent disjunction results in gametes carrying unbalanced chromosome complements, early arrest is clearly associated with aneuploid eggs. We infer that most chromosomes carry genes requisite to normal embryonic development.
Table 1 represents a revision and update of linkage groupchromosome correlations. Correlation of linkage group V with chromosome 1 I is based on linkage of a group V marker, narrow-abdomen (nu. with T( 11; 12)/l I; 12 and independence of na from a chromosome I2 marker, pale-body (Ph) (Ross, unpublished data). Reidentification, noted above. of T(9;l I) as T(8;9)/8;9 indicates that linkage group 6 is on chromosome 8 rather than chromosome 6. T(4;6)/4;6 CM shares chromosome 4 with a progres- sive ring-of-six, T(4;5;10)/4:5;10, but a group X marker, yellow-body 0,). is linked only with T(4;5;10)/4;5;10. Since J is independent of the chromosome 10 marker, ro, group X is correlated with chromosome 5. By elimination. the sole marker of linkage group II, balloon-wing (ha), apparently lies on chromosome 2, a chromosome not involved in any of the translocations. While the number of loci in the three main linkage groups, VI. VIII and X, has grown, identification of other groups remains
Table 1. Linkage group-chromosomr correlations
II III IV V
I (Xl ?
4 or 6
9 3 5
4 or 6
Cockroach genetics 553
dependent on one or two mutant markers and/or chromosome breakpoints. Completion of the linkage group-chromosome correlations and a renumbering of the two systems remains a goal of this work.
When the proportion of viable embryos within an egg case is quite low, those embryos may not be able to force the egg case open at the time when hatch normally occurs. This embryonic trapping is a unique sterility mechanism. Phenotypically distinct males carrying a sufficiently high lethality to cause complete egg case sterility in matings to wild type females were synthesized by combining T(8;9)/3;9 with T(4;5;10)/4;5;10. A lower lethality in matings of T(4;5;10)/4;5;10 females than males made this combination possible. The males were equally competitive with wild type. A pilot field test with the sterile males was partially successful against a population that had been reduced by an insecticide treatment. High sterility occurred in localized groups, near to which the males were released. However, uncontrolled growth in new groups that were appar- ently established due to insecticide-induced dispersal increased the size of the total population.
Intra- and inter-speciJic chromosomal polymorphism
Frequent fusions between heterochromatic regions on autosomes, chromosome fragments, and an aberrant chiasma frequency distinguish a multi-high level resistant field strain from a susceptible field strain (Ross, 1986). One male of the resistant strain was heterozygous for a reciprocal translocation and a spontaneous translocation was found in one cell of another male. These findings support a hypothesis of a relationship between fusion-breakage and chromo- some rearrangements. The translocation is the second to be found in a field strain. Study of additional strains is needed to determine whether translocations are the most common type of floating chromoso- ma1 polymorphism in B. germanica, and whether a tendency for fusion of heterochromatin is in some way related to selection pressure from insecticides.
The only species with which B. germanica is known to hybridize is the Asian cockroach, B. asahznai Mizukubo, that was introduced recently into the USA. Other attempts to obtain interspecific hybrids of the German cockroach with closely related species and, indeed, among other cockroach groups have been unsuccessful. The only cytotaxonomic difference found between B. germanica and B. asahinai is attributable to a non-reciprocal translocation, whereby the nucleolar organizing region (NOR) on the X chromosome of B. germanica was shifted to the end of the short arm of chromosome 12 of B. asahinai (Ross, 1988). Except for this difference and a visibly smaller size of the X in B. asahinai than in B. germanica, gross chromosome morphology is essen- tially identical in the two species. Hybrid males are heterozygous for a NOR-bearing segment on chromosome 12, but otherwise the autosomes pair closely throughout their lengths. Since the sex mechanism is X0, hybrid males from B. germanica females inherit a typical German-type X chromosome and those from B. asahinai a typical Asian-type X. Comparisons of cytological differences and species-
specific traits are being used to test a hypothesis that the translocation was the primary mechanism of species divergence.
Aneuploid eggs from matings of translocation heterozygotes show an array of gross defects, rather than simple cessations of normal development. Certain types, such as a lack of control over the position of the germ anlage in eggs of matings of T(7; 12)/7; 12, are specific to certain translocations and/or chromosomes. If unbalance of certain chromosomes has indeed upset systems regulating early development, the cockroach eggs could be extremely valuable to investigations on the genetic control of early development.
Post embryonic studies on mutants also yield information of interest to developmental and evolutionary genetics. Examples include studies of the chromosome 9 mutants mentioned earlier. Evidence of modifiers acting throughout develop- ment was found in experiments with sty (Tanaka and Ross, 1989). Patterns of mottling on a maternally-in- herited mutant, mottled-body (Mt), may give insight into random vs non-random cleavage divisions. Whenever the abdomen is divided into two regio...