Universidade Federal do Amapá, Campus Binacional, Oiapoque, Amapá, Brazil.
School of Life Sciences, Arizona State University, Tempe, AZ, USA.
Sci Rep. 2022 Mar 22;12(1):4860. doi: 10.1038/s41598-022-08537-x.
The prevalent mode of reproduction among ants is arrhenotokous parthenogenesis where unfertilized eggs give rise to haploid males and fertilized eggs develop into diploid females. Some ant species are capable of thelytokous parthenogenesis, a type of asexual reproduction where females develop from unfertilized diploid eggs. Thelytoky is well-documented in more than 20 ant species. Cytogenetic data are available for six species demonstrating that some thelytokous ant species are capable of producing males occasionally as well as maintaining their chromosome numbers and proper chromosome pairings. Mycocepurus smithii is a thelytokous fungus-growing ant species that inhabits large parts of Central and South America. Cytogenetic data are unavailable for M. smithii and male individuals were never documented for this species, although the presence of males is expected because genetic recombination was observed in a few sexually reproducing populations in Brazil and haploid sperm was documented from the spermathecae of M. smithii queens. This study aims at comparatively studying asexual and sexual populations of M. smithii using classical and molecular cytogenetic methods to test whether karyotype configuration is modified according to the mode of reproduction in M. smithii. Moreover, we report the discovery of M. smithii males from a sexually reproducing population in the Brazilian state Pará, diagnose the male of M. smithii, and morphologically characterize their spermatozoa. Karyotypic variation was observed within the asexual population (2n = 9, 10, or 11), whereas the chromosome number was fixed in the sexual population (2n = 14, n = 7). Identical karyotypes were maintained within individual M. smithii colonies and karyotype variation was only observed between colonies. In asexual individuals, the karyomorphs showed a decay of homologous chromosome pairs, especially in individuals with the karyomorph 2n = 11, which is potentially caused by relaxed natural selection on proper chromosome pairing. In contrast, females in the sexual population showed proper homologous chromosome pairings. In individuals of both asexual and sexual populations, we find that heterochromatin was localized in centromeric regions and on the short arms of the chromosomes, GC-rich regions were associated with heterochromatic regions, and 18S rDNA genes were located on the largest chromosome pair. This comparative cytogenetic analysis contributes to our understanding about the cytological mechanisms associated with thelytokous parthenogenesis in ants and suggests the decay of chromosome structure in the absence of meiosis and genetic recombination.
蚂蚁中普遍的繁殖方式是孤雌生殖，其中未受精的卵产生单倍体雄性，而受精的卵则发育成二倍体雌性。一些蚂蚁物种能够进行产雌孤雌生殖，这是一种无性繁殖方式，其中雌性由未受精的二倍体卵发育而来。产雌孤雌生殖在超过 20 种蚂蚁中得到了很好的记录。已有 6 种蚂蚁的细胞遗传学数据表明，一些产雌孤雌生殖的蚂蚁物种偶尔也能够产生雄性，并保持其染色体数量和正确的染色体配对。Mycocepurus smithii 是一种产雌真菌蚁种，栖息在中美洲和南美洲的大部分地区。目前尚无关于 M. smithii 的细胞遗传学数据，也从未记录过该物种的雄性个体，尽管预计会存在雄性个体，因为在巴西的一些有性繁殖种群中观察到了遗传重组，并且从 M. smithii 蚁后的精囊中记录到了单倍体精子。本研究旨在使用经典和分子细胞遗传学方法比较研究 M. smithii 的无性和有性种群，以检验 M. smithii 的生殖方式是否会改变其核型结构。此外，我们还报告了在巴西帕拉州的一个有性繁殖种群中发现了 M. smithii 的雄性个体，诊断了 M. smithii 的雄性，并对其精子进行了形态学特征描述。在无性种群中观察到了核型变异（2n=9、10 或 11），而在有性种群中染色体数量是固定的（2n=14，n=7）。在单个 M. smithii 群体中保持了相同的核型，仅在群体之间观察到核型变异。在无性个体中，同源染色体对的退化现象明显，尤其是在核型为 2n=11 的个体中，这可能是由于对正确染色体配对的自然选择放松所致。相比之下，有性种群中的雌性表现出正确的同源染色体配对。在无性和有性种群的个体中，我们发现异染色质定位于着丝粒区域和染色体的短臂上，富含 GC 的区域与异染色质区域相关，18S rDNA 基因位于最大的染色体对上。这项比较细胞遗传学分析有助于我们了解与蚂蚁产雌孤雌生殖相关的细胞学机制，并表明在没有减数分裂和遗传重组的情况下，染色体结构的退化。
