ROBERTSONIAN POLYMORPHISMS REDUCE RECOMBINATION ALONG A GEOGRAPHICAL CLINE IN THE WATERHYACINTH GRASSHOPPER, Cornops aquaticum.

COLOMBO, P.C.

Foz do Iguaçu

Congreso; Congreso Latinoamericano de Genética; 2006

Sociedade Brasileira de Genêtica

C. aquaticum (Leptysminae: Acrididae) lives in close connection with waterhyacinths from southern Mexico to Argentina. The blue-flowered waterhyacinth, Eichhornia crassipes, has been introduced elsewhere as an ornamental plant and, without natural enemies, it has become a weed. Recently C. aquaticum has been considered as a possible biological control of >E. crassipes, and hence it has become a global focus of attention. However, its cytogenetics has remained largely ignored. In this work six Argentinian populations and a Caribbean one were sampled and cytologically studied; from north to south: Trinidad and Tobago (13 males), Corrientes (46), Santa Fe (42), Rosario (19), San Pedro (12 males and 19 females), Zárate (27 males and 19 females) and Tigre (14 males). Testes were dissected and fixed in 3 ethanol: 1 acetic acid; female gut caeca were dissected upon injection of colchicine 0.5 %. Cytological analysis was performed by squash in proprionic haematoxylin. Male meiosis and female C-metaphases were studied; chiasmata were registered in 10 metaphase I plates individual and classified as proximal, interstitial or distal to the centromere. Data were processed with the STATISTICA package. C. aquaticum has 2n= 23 chromosomes in males and 24 in females, with a X0/XX sex determination system. All chromosomes are acro-telocentric and were numbered from 1 to 11 according to their size. Chromosomes 9 to 11 are distinctly small; the others range from large to medium-sized. Upon this basic karyotype three Robertsonian rearrangements (=centric fusions) took place between pairs 1 and 6 (fusion 1/6), 2 and 5 (2/5) and 3 and 4 (3/4). These polymorphisms are restricted to the lower course of the Paraná river, between Rosario and Buenos Aires, in an area comprising less than 300 kms. Trinidad and Tobago and Corrientes are monomorphic, without fusions; Santa Fe has 0.12 fusions per individual (fpi); in Rosario this value keeps low (0.11) and then it raises spectacularly southwards (San Pedro: 1.25 fpi; Zarate: 3.07 fpi). Tigre, the southernmost population, with 5.14 fpi, is almost monomorphic for fusions. In San Pedro and Zarate the fusion karyotypes, when considered separately, were mostly in keeping with Hardy-Weinberg values; when considered pairwise, they showed to be in gametic phase equilibrium. The rearrangements cause a drastic chiasma repatterning in the fusion bivalents (or trivalents) reducing proximal and interstitial chiasma frequency. Recombination is also reduced due to the loss of independent segregation. A recombination index that takes into account both factors (Tostoand Bidau 1987) correlates negatively with the number of pairs affected by fusions among populations. The problem of why the fusions are cramped in such a small area in a species of Neotropical distribution has no easy explanations. Much more research is needed in order to shed light on this issue.