Mecanismos de diferenciação cromossômica em besouros da subfamília Cassidinae s.l. (Polyphaga, Chrysomelidae)
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Data
2016-04-04
Autores
Lopes, Amália Torrezan [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
Os besouros da subfamília Cassidinae s.l. são popularmente conhecidos como “tortoise beetles” devido ao formado peculiar dos élitros que se assemelham ao casco de tartaruga. Esta subfamília inclui mais de 6000 espécies descritas taxonomicamente, das quais menos de 2% foram estudadas sob o ponto de vista citogenético, e evidenciaram número diploide variando entre 2n=16 a 2n=51 e sistemas cromossômicos sexuais dos tipos Xyp, X0, Xy, Xyc, Xyr, neoXY, Xyyp, XpneoXneoYp, entre outros sistemas múltiplos. No entanto, cerca de 85% das espécies apresentaram predominância do cariótipo 2n=16+Xyp, e esta característica cariotípica pode estar relacionada ao baixo número de espécies examinadas, a predominância de análises em espécies pertencentes a um mesmo gênero e/ou tribo, bem como a escassez ou até mesmo a inexistência de estudos que utilizaram marcação de regiões cromossômicas específicas. O objetivo deste trabalho é discutir sobre os mecanismos de evolução cromossômica em espécies de Cassidinae. As análises cromossômicas foram realizadas em 19 espécies de cassidíneos da fauna brasileira, através de coloração convencional, bandeamento C e hibridação in situ fluorescente (FISH) com sondas dos genes ribossomais 28S, 5S, dos clusters teloméricos (TTAGG)n e (TCAGG)n, e do pequeno RNA nuclear (snRNA) U2. O estudo citogenético de 19 espécies de Cassidinae revelou uma grande diversidade em relação ao número diploide - Cassidini: 2n=38+Xyp em Agroiconota inedita, 2n=20+Xyp em Charidotella immaculata, Charidotella sexpunctata e Microctenochira stigmatica, 2n=16+Xyp em Deloyala cruciata, Microctenochira aciculata, Microctenochira optata e Microctenochira quadrata, 2n=18 em Microctenochira gnata; Goniocheniini: 2n=16+Xyp em Chlamydocassis cribripennis; Ischyrosonychini: 2n=16+Xyp em Cistudinella obducta; Mesomphaliini: 2n=34+Xyp em Botanochara tesselata, 2n=20+Xyp/Xyr em Chelymorpha cribraria, 2n=20+Xyp em Chelymorpha inflata, 2n=20 em Cteisella magica, 2n=38+Xyp em Cyrtonota cyanea, 2n=40+Xyp em Paraselenis flava, 2n=22 em Stolas areolata, e 2n=24+Xyp em Stolas redtembacheri. O cariótipo 2n=16+Xyp ocorreu em 26% das espécies analisadas pertencentes as tribos Cassidini, Goniocheniini e Ischyrosonychini. O sistema cromossômico sexual (SCS) Xyp foi o mais frequente, sendo registrado inclusive em espécies da tribo Mesomphaliini, que caracteristicamente possui uma grande diversidade de SCS. Em relação ao padrão de heterocromatina constitutiva, 12 espécies exibiram cromossomos com marcações pericentroméricas, as quais estavam presentes em alguns ou até mesmo todos os elementos do conjunto diploide. Adicionalmente, quatro espécies revelaram blocos heterocromáticos nas regiões intersticial e terminal de alguns cromossomos autossômicos e/ou sexuais. O gene de rDNA 28S exibiu um padrão similar quanto ao número de clusters no cariótipo, uma vez que 10 das 11 espécies examinadas, tanto com cariótipos semelhantes quanto diferentes, exibiram dois sítios ribossomais. Entretanto, houve diferenças quanto a localização dos sítios de rDNA, especialmente em representantes de Cassidini, os quais exibiram a maior homogeneidade cariotípica. Entre as 12 espécies investigadas quanto a presença dos clusters teloméricos (TTAGG)n e (TCAGG)n, apenas nove mostraram marcações positivas para a sequência (TTAGG)n, porém com variações no número de cromossomos portadores do sítio telomérico. Este resultado demonstra que, provavelmente, esta sequência telomérica está sendo repetidamente perdida ao longo da evolução cromossômica desta subfamília. A FISH com sonda de rDNA 5S não revelou sinais de hibridação positivos em todas as espécies investigadas, com exceção de Cha. immaculata, que exibiu um sinal tênue de marcação em núcleos interfásicos. O gene do snRNA U2 foi visualizado apenas em um par autossômico, mas com variação quanto a localização entre as espécies.
The beetles of the subfamily Cassidinae s.l. are commonly known as tortoise beetles, due to the peculiar shape of the elytra. This subfamily possesses more than 6000 taxonomically described species, of which less than 2% were cytogenetically studied. The cassidines showed diploid number ranged from 2n=16 to 2n=51, and sex chromosome systems of the Xyp, X0, Xy, Xyc, Xyr, neoXY, Xyyp, XpneoXneoYp types, in addition to other multiple systems. Despite the occurrence of this karyotype diversity, about 85% of the species presented the karyotype 2n=16+Xyp. However, this karyotype homogeneity may be related to the low number of species examined, the analyses with species included in the same genus and/or tribe, as well as the scarcity or lack of studies with techniques that identify specific chromosomal regions. The aim of this work is to discuss about the mechanism of chromosome evolution in species of Cassidinae. The chromosomal analysis were performed in 19 species of Cassidinae from Brazilian fauna, using standard staining, C-banding and fluorescent in situ hybridization (FISH) with probes of 28S rDNA, 5S rDNA, (TTAGG)n and (TCAGG)n telomeric clusters, and U2 small nuclear RNA (snRNA). The cytogenetic studies in 19 cassidine beetles revealed a high diversity of diploid number - Cassidini: 2n=38+Xyp in Agroiconota inedita, 2n=20+Xyp in Charidotella immaculata, Charidotella sexpunctata and Microctenochira stigmatica, 2n=16+Xyp in Deloyala cruciata, Microctenochira aciculata, Microctenochira optata and Microctenochira quadrata, 2n=18 in Microctenochira gnata; Goniocheniini: 2n=16+Xyp in Chlamydocassis cribripennis; Ischyrosonychini: 2n=16+Xyp in Cistudinella obducta; Mesomphaliini: 2n=34+Xyp in Botanochara tesselata, 2n=20+Xyp/Xyr in Chelymorpha cribraria, 2n=20+Xyp in Chelymorpha inflata, 2n=20 in Cteisella magica, 2n=38+Xyp in Cyrtonota cyanea, 2n=40+Xyp in Paraselenis flava, 2n=22 in Stolas areolata, and 2n=24+Xyp in Stolas redtembacheri. The karyotype 2n=16+Xyp ocurred in 26% of the analyzed species, belonging to the tribes Cassidini, Goniocheniini and Ischyrosonychini. The Xyp sex chromosome system (SCS) was the most frequent, occurring also in Mesomphaliini that exhibit the highest diversity of SCS within the Cassidinae. In relation to the pattern of constitutive heterochromatin, 12 species showed chromosomes with pericentromeric bands, which were present in some or all elements of the diploid set. Additionally, four species revealed heterochromatic blocks in the interstitial and terminal regions of some autosomal and/or sex chromosomes. In 10 of the 11 species investigated, the 28S ribosomal gene showed a similar pattern, occurring in only one autosomal pair. However, differences regarding the localization of the 28S rDNA sites were observed among the species, mainly in representatives of Cassidini, which showed the highest karyotype homogeneity. Among the 12 species studied for the presence of the (TTAGG)n and (TCAGG)n telomere clusters, only nine showed (TTAGG)n positive signals, but with variations regarding the number of chromosomes with the telomere sites. This result probably indicates that the TTAGG telomere sequence have repeatedly been lost during the chromosome evolution of this subfamily. The FISH with 5S rDNA probes did not revealed positive sites in all investigated species, with the exception of Cha. immaculata that exhibited a tenuous signal in interphase nuclei. The U2 snRNA gene was visualized in only one autosomal pair, but with variation in chromosomal location among the species.
The beetles of the subfamily Cassidinae s.l. are commonly known as tortoise beetles, due to the peculiar shape of the elytra. This subfamily possesses more than 6000 taxonomically described species, of which less than 2% were cytogenetically studied. The cassidines showed diploid number ranged from 2n=16 to 2n=51, and sex chromosome systems of the Xyp, X0, Xy, Xyc, Xyr, neoXY, Xyyp, XpneoXneoYp types, in addition to other multiple systems. Despite the occurrence of this karyotype diversity, about 85% of the species presented the karyotype 2n=16+Xyp. However, this karyotype homogeneity may be related to the low number of species examined, the analyses with species included in the same genus and/or tribe, as well as the scarcity or lack of studies with techniques that identify specific chromosomal regions. The aim of this work is to discuss about the mechanism of chromosome evolution in species of Cassidinae. The chromosomal analysis were performed in 19 species of Cassidinae from Brazilian fauna, using standard staining, C-banding and fluorescent in situ hybridization (FISH) with probes of 28S rDNA, 5S rDNA, (TTAGG)n and (TCAGG)n telomeric clusters, and U2 small nuclear RNA (snRNA). The cytogenetic studies in 19 cassidine beetles revealed a high diversity of diploid number - Cassidini: 2n=38+Xyp in Agroiconota inedita, 2n=20+Xyp in Charidotella immaculata, Charidotella sexpunctata and Microctenochira stigmatica, 2n=16+Xyp in Deloyala cruciata, Microctenochira aciculata, Microctenochira optata and Microctenochira quadrata, 2n=18 in Microctenochira gnata; Goniocheniini: 2n=16+Xyp in Chlamydocassis cribripennis; Ischyrosonychini: 2n=16+Xyp in Cistudinella obducta; Mesomphaliini: 2n=34+Xyp in Botanochara tesselata, 2n=20+Xyp/Xyr in Chelymorpha cribraria, 2n=20+Xyp in Chelymorpha inflata, 2n=20 in Cteisella magica, 2n=38+Xyp in Cyrtonota cyanea, 2n=40+Xyp in Paraselenis flava, 2n=22 in Stolas areolata, and 2n=24+Xyp in Stolas redtembacheri. The karyotype 2n=16+Xyp ocurred in 26% of the analyzed species, belonging to the tribes Cassidini, Goniocheniini and Ischyrosonychini. The Xyp sex chromosome system (SCS) was the most frequent, occurring also in Mesomphaliini that exhibit the highest diversity of SCS within the Cassidinae. In relation to the pattern of constitutive heterochromatin, 12 species showed chromosomes with pericentromeric bands, which were present in some or all elements of the diploid set. Additionally, four species revealed heterochromatic blocks in the interstitial and terminal regions of some autosomal and/or sex chromosomes. In 10 of the 11 species investigated, the 28S ribosomal gene showed a similar pattern, occurring in only one autosomal pair. However, differences regarding the localization of the 28S rDNA sites were observed among the species, mainly in representatives of Cassidini, which showed the highest karyotype homogeneity. Among the 12 species studied for the presence of the (TTAGG)n and (TCAGG)n telomere clusters, only nine showed (TTAGG)n positive signals, but with variations regarding the number of chromosomes with the telomere sites. This result probably indicates that the TTAGG telomere sequence have repeatedly been lost during the chromosome evolution of this subfamily. The FISH with 5S rDNA probes did not revealed positive sites in all investigated species, with the exception of Cha. immaculata that exhibited a tenuous signal in interphase nuclei. The U2 snRNA gene was visualized in only one autosomal pair, but with variation in chromosomal location among the species.
Descrição
Palavras-chave
Cariótipo, Genes ribossomais, Heterocromatina constitutiva, Meiose, Sistema cromossômico sexual, Telômero