Cytogenetic analysis in the terrestrial orchid Sacoila argentina (Griseb.) Garay from Paraguay

Abstract

Sacoila argentina (Griseb.) Garay is a terrestrial orchid with ornamental value found in northern Argentina, Bolivia, and Paraguay and is considered an object of conservation within the Paraguayan Chaco region, only found at Boquerón Department and chromosomally unknown until this study. Here, we present another natural population of Sacoila argentina for Paraguay (Paraguarí Department, Villa Florida, 57º07’48.71” W - 26º23’32.16” S) so extending its area of distribution. Conventional cytogenetic techniques were applied in order to analyze its genetic system. Sacoila argentina is a diploid species with 2n = 2x = 46 small size chromosomes (1.21 - 4.45 µm) and 40.63 µm per haploid genome. Its karyotype, composed of 38m + 8sm, is bimodal (A2=0.36/R=3.69) due to a pair (# 20, sm) that is more than two times larger than the mean length of the chromosome (1.77 µm), and symmetric (A1=0.18/i=44.58/r>2=0.13) belonging to Stebbins category 2B. The chromosome pair # 20 also carries a terminal macrosatellite on its short arm with an active NOR. Sporogenesis is normal and pollen grain viability is high (80%). Meiotic behaviour is regular and chromosomes pair up as 23 bivalents in the male (pollen mother cells, PMC) as they do in the female meiosis (megaspore mother cells, MMCs), confirming the basic number x = 23 for Sacoila. In PMCs and MMCs at diakinesis/metaphase I, ring bivalents (80%) with distal chiasmata (85%) are common, and the mean of chiasmata per bivalent is high (1.80) in both germinal lines. The recombination index, affected by the meiotic number (n = 23) and the mean of chiasmata/cell (41.5) is high (RI=64.5). Natural populations of Sacoila argentina possess low density, are scarce, interspersed and just found at flowering time. For these reasons, it is a rarely collected species.