Mating system and two types of gametogenesis in the fresh water diatom Ulnaria ulna (Bacillariophyta)

Abstract

Sexual reproduction and mating system of the freshwater diatom Ulnaria ulna (Nitzsch) Compère were studied by using clonal cultures. Mating system of the species involves homo-and heterothallic modes of reproduction; and both male and female clones were capable of homothallic reproduction. Two types of gametogenesis corresponding to two mating types were investigated. Analysis of the crossing table, gamete morphology, and sex distribution in the progeny resulted from intraclonal reproduction provided evidences that anisogamy and two mating types were determined in U. ulna genetically; male and female clones were hetero-and homogametic correspondently. Specific active movement of male gametes caused by the formation and retraction of pseudopodia-like structures on the gamete surface was described. The absence of reproductive isolation between clones gathered from geographically distant populations suggests continuity and broad distribution of the species.

Reproductive properties of diatoms significant for their cultivation and biotechnology

“…In pennate diatoms, the sex is genetically deter mined and, judging from the available data, pennates follow MF/FF inheritance pattern, where male sex is determined by MF combination of sexual factors and female sex is defined by FF combination; in other words, male sex is heterogametic [17,[25][26][27][28]. Heter ogametic nature of the male sex was for the first time proved in the course of investigation of the breeding system in Nitzschia longissima (Brébisson) Grunov [25].…”

“…For instance, in Ulnaria ulna (Nitzsch) Compére heterothallism is associated with a normal type of sexual process, whereas homothallic reproduc tion occurs by means of paedogamy (a sort of auto mixis) [28]. Most often, the system of crossing com bines both pathways with greater or lesser dominance of one of them [39].…”

Sexual reproduction in plagiogrammacean diatoms: First insights into the early pennates

“…In araphids, amphimixis includes oogamy with small, motile spermatia and large immotile eggs ( Rhabdonema and Grammatophora ; [ 18 , 27 , 93 , 94 ]), physiological and behavioural anisogamy of more or less equal size gametes, each with sex-specific behaviour and morphology [ 19 – 21 , 95 ], and strict isogamy ( Meridion circulare , at least in some reports; [ 95 , 96 ]). The most commonly reported thus far is anisogamy: behavioural (exemplified in species such as Licmophora communis [ 97 ]), and behavioural and morphological, exemplified by Tabularia fasciculata [ 20 ] or Ulnaria ulna [ 23 ].…”

“…From these and earlier works [ 2 , 17 – 19 ], it has become evident that araphid pennates evolved some very unusual types of sex cell and auxospore structures that are not shared with centric or pennate diatoms. For example, a unique form of oogamy is known in species of the genus Rhabdonema [ 18 ], and non-flagellated but vigorously motile male gametes in Tabularia , Pseudostaurosira , and Ulnaria [ 20 – 23 ], among other characters. This leads to the question of whether any of these unusual characters were present in the earliest of the araphid pennates, or if they are restricted to the more recently diverged members.…”

“…In this light, it is apparent that all the modern studies of diatom sexuality and reproductive structures to date include only the members of more divergent, “core” araphid families (Fragillariaceae, Tabulariaceae, Rhabdonemaceae, Licmophoraceae sensu reference [ 27 ], etc. ; [ 13 – 15 , 20 , 23 , 36 ]). Reproductive biology, sex cell structure and their behaviour have not yet been investigated in any member of the "basal” araphids, including those in Rhaphoneidaceae, Plagiogrammaceae and Asterionellopsis -like species.…”