Heredity 58 (1987) 341-343 The Genetical Society of Great Britain Received 6 June 1986

New hypotheses about the origin of supernumerary chromosome segments in

J. P. M. Camacho and Departamento de BiologIa , Ecologla y J. Cabrero Genética, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain.

The existence of euchromatic supernumerary chromosome segments which do not seem to represent repeated material cannot be explained by the current hypotheses (duplication or translocation) concerning the origin of extra segments. An hypothesis is proposed to explain the origin of the euchromatic extra segments distally located on the megameric M6 chromosome of the species Omocestus bolivari and binotatus, namely that these segments represent a relic of an euchromatic region which formed part of the ancestral M6 chromosome of the gomphocerine grasshoppers. On the other hand, the absence of paracentromeric C-bands in some chromosomes of Chorthippus vagans and Ch. brunneus could have been achieved by means of transformation of heterochromatin into euchromatin.

INTRODUCTION cytogenetic techniques employed for analysing these materials were the same as in Camacho et Twomechanisms have been proposed for the a!. (1984). origin of the supernumerary heterochromatic seg- ments (White, 1954, 1973), both involving the repe- tition of chromosome segments. Firstly, by trans- location from a chromosome of the standard RESULTS complement or a B chromosome. Secondly, by amplification of some part of the genome. Euchorthippus pulvinatus (fig. 1A) Although the second hypothesis is accepted by the majority of authors (Shaw, 1971; John, 1973; While the M6 chromosome carries a subdistally Hewitt, 1979; Camacho et a!., 1984), we have located C-band in the majority of Spanish popula- recently obtained some evidence for the origin of tions analysed (Santos and Giráldez, 1982), one an extra segment in the Qedipoda fus- male from the Sierra de Alfacar (Granada) was cocincta by translocation from the megameric M9 heterozygous for the presence/absence of such chromosome (Camacho et al., 1986). C-band. On the other hand, the 58 is frequently In this paper we analyse a series of supernu- polymorphic for a supernumerary segment located merary segments including two new forms of proximally and darkly C-banded (Santos, 1980; chromatin variation, one involving euchromatin Riva et aL, 1984). We have found it in one out of and the other heterochromatin, whose origin needs 13 females analysed from Maitena (Granada). to be explained by means of new hypotheses. Stenobothrusfestivus (fig. 18) TheM7 and S8 chromosomes may carry distal supernumerary segments which C-band darkly, MATERIALSAND METHODS although that on the M7 is much larger than that on the S8. Severalnatural populations of the grasshopper species Euchorthippus pulvinatus, Stenobothrus fes- Omocestusraymondi (fig. 1C) tivus, Omocestus raymondi, Chorthippus apicalis, Ch. binotatus, C/i. vagans and Ch. brunneus were Thisspecies shows variation for the size of the sampled in the South of the Iberian Peninsula. The interstitial C-band in the X chromosome which 342 J. P. M. CAMACHO AND J. CABRERO coincideswith the location of the nucleolar

organiser region in this species (Cabrero and e .55 Camacho, 1986b). Furthermore, there was vari- S .I. ation for the presence of two types of darkly C- .

banded extra segments located distally on the M7 —5 chromosome. > w C)

Chorthippusapicalis (fig. 1D) a)

Adarkly C-banded supernumerary segment • — located distally on the M7 chromosome was found -—

in Alcalá la Real population with the following 1 —e UI a

karyomorphic frequencies: 4BB, 8Bs, iss. r (I) a T1 m Chorthippusbinotatus (fig. 1E) Wehave found eight different supernumerary seg- ments in Spanish populations of this species

(Cabrero et a!., 1986). Here we will mention only S S. the euchromatic segment located distally on the S

M6 chromosome. ,Is . r x

Chorthippusvagans (fig. iF) —s PS- Inthis species, we have found C-band variation I in three chromosome pairs: L3, M4 and M5. Some •

L3 chromosomes possess an interstitial extra C- II band in their short arm while some M4 and M5 chromosomes lack the paracentromeric C-band. UI

Chorthippus brunneus (fig. iG) C) Figure 1 Supernumerary segments and C-band variations in Themajority of populations analysed were poly- gomphocerine grasshoppers. (A) Euchorthippuspulvinatus: morphic for C-band variations and supernumerary unequal M6 bivalent and heteromorphic S pair. (B) Stenobothrus festivus: unequal M7 and S bivalents. (C) chromosome segments. With the single exceptior Otnocestus raymondi: heteromorphic X chromosomes from of the L3, all remaining chromosomes show some a female embryo, M7 chromosome with a small distal extra type of variation. A more detailed examination of segment and M7 bivalent with an unsegmented member such a complex polymorphism will be made in a and the other carrying a large extra segment. (D) Chorthip- separate paper. The variations in heterochromatin pus apicalis: M7 chromosome with a distal extra segment. (E) Chorthippus binotatus: euchromatic extra segment dis- found in this species can be classified into three tally located on C-banded (left) and silver stained (right) types: (a) variation for the size of paracentromeric heteromorphic M6 bivalents. (F) Chorthippus vagans: C- C-bands in the L1 and L2 chromosomes, (b) band variations in L3, M4 and M5 chromosomes. Note the amplification of C-bands in the L2 associated with presence in the L3 on the right of an extra C-band beyond the interstitial C-band on the short arm, and the absence a NOR, M7 and S8 chromosomes, and (c) addi- of paracentromeric C-band in M4 and M5 homologues on tional C-bands, including small C-bands located the right. (G) Chorthippus brunneus: supernumerary seg- interstitially on the X, M4 and M5 chromosomes, ments and C-band variations in chromosomes from and larger C-bands on the M4, M5 and M6 neuroblast embryo mitoses. L1: variation in size of paracen- tromeric C-band. L2: variation for the size of interstitial chromosomes. C-band on short arm and for presence or absence of paracentromeric C-band. X: interstitial extra C-band. M4: interstitial extra C-bands and subdistal extra segment. M5: DISCUSSION interstitial and distal extra C-bands. M6: from left to right, basic chromosome, one lacking paracentromeric C-band, Theamplification hypothesis could explain the one carrying a grey extra segment and one possessing two darkly C-banded extra segments. M7: variation for the size origin of the majority of supernumerary of the distal C-band. S8: basic (left) and segmented heterochromatic chromosome segments analysed chromosomes. ORIGIN OF SUPERNUMERARY CHROMOSOME SEGMENTS 343 in this report, but it is especially consistent with REFERENCES the amplification of pre-existing C-bands observed in the S5 of E.pulvinatus, the X of O.raymondi and CABRERO,.t. 1985. Estudios citogenéticos en saltamontes de Ia subfamilia : Heterochromatina, reor- the L2, M7 and S8 of Ch.brunneus. When an denaciones cromosómicas y actividad nucleolar. Tesis heterochromatic supernumerary segmentis Doctoral, Universidad de Granada. located in a chromosome region which is euchro- CABRERO, J. AND CAMACHO, J. P. M. 1986a. Cytogenetic matic in its partner chromosome, the amplification studies in gomphocerine grasshoppers. I. Comparative hypothesis needs to be coupled with the hetero- analysis of chromosome C-banding pattern. Heredity, 56, 365—372. chromatinisation of the duplicated regions (John, CABRERO, J, AND CAMACHO, 3. P. M. 1986b. Cytogenetic 1973). However, two other types of chromatin vari- studies in gomphocerine grasshoppers. II. Chromosomal ation found in our investigation seem to have location of active nucleolar organizing regions. Can. J. arisen by other mechanisms, namely the disappear- Genet. CytoL, 28, 540-544. CABRERO, .1., NAVAS-CASTILLO, 3. AND CAMACHO, J. P. M. ance of paracentromeric C-bands and the 1986. Effects of supernumerary chromosome segments on existence of euchromatic extra segments. On the the activity of nucleolar organiser regions in the grasshop- one hand, the paracentromeric C-bands are absent per Chorthippus binotatus. Chromosoma, 93, 375-380. from some chromosomes of Ch.vagans and CAMACHO, J. P. M., NAVAS-CASTILLO, J. AND CABRERO, J. Ch.brunneus. Because the presence of paracen- 1986. Extra nucleolar activity associated with presence of a supernumerary chromosome segment in the grasshopper tromeric C-bands is the norm in gomphocerine Qedipoda fuscocincta. Heredity, 56,237-241. chromosomes (Cabrero and Camacho, 1986a) and CAMACHO, 3. P. M., VISERAS, E., NAVAS, 3. AND CABRERO, J. given that there is no change in morphology or 1984. C-heterochromatin content of supernumerary size of these chromosomes (as would be expected chromosome segments of grasshoppers: detection of an euchromatic extra segment. Heredity, 53, 167—175. in the case of a deletion), we propose that these HEWITT, C. M. 1979. Grasshoppers and crickets. Animal paracentromeric C-bands have been lost by means Cytogenetics 3, Insecta 1 . Gebrüder Born- of a process of transformation of heterochromatin traeger, Berlin-Stuttgart, l7Opp. into euchromatin, similarly to that reported for the JOHN, B. 1973. The cytogenetic systems of grasshoppers and paracentromeric C-bands in the neo-Y chromo- locusts. II. The origin and evolution of supernumerary segments. Chromosoma, 44, 123-146 some of the grasshopper Slenacatantops angusti- KING, M. AND JOHN, B. 1980. Regularities and restrictions frons (King and John, 1980). On the other hand, governing C-band variation in Acridoid grasshoppers. the existence of euchromatic extra segments in the Chromosoma, 76,123-150. M6 chromosomes of Chorthippus binotatus (this RIVA, E., FOX. D. P. AND SANTOS, 3. L. 1984. 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(the megameric M6) seems to indicate that they Cambridge Univ. Press, London and New York. could actually be a relic of an euchromatic region which formed part of the ancestral M6 chromo- some of the gomphocerine grasshoppers, a region which presumably has been lost in species other than O.bolivari and Ch.binotatus.